Rhodium

Rhodium-plated silver jewelry ©Tkachenkoproduction – stock.adobe.com

Rhodium is a rare precious metal that does not rust. It is best known for its use in automotive catalytic converters but is also employed in the chemical industry for catalytic reactions. Due to its corrosion resistance and high reflectivity, it is also used in various ranges of technologies.

 

How can I come into contact with this material?

In everyday life, rhodium is usually found not as a pure metal but in alloys or as a coating. It enters the environment primarily through abrasion from automotive catalytic converters, usually in very small quantities, such as fine dust.

Other examples include jewelry and technical or analytical equipment. Direct contact with these items usually occurs in specialized laboratories or workshops.

Is there any risk from this material to humans and the environment?

Due to its low reactivity, elemental rhodium poses little risk to human health or the environment.

In contrast, the release of rhodium compounds or fine particles during manufacturing, processing, or disposal activities increases risk. Exposure via direct skin contact or inhalation may cause irritation or adverse health effects.

Conclusion

Rhodium is found primarily in industrial settings and may enter the environment through sources such as road traffic.

Based on current knowledge, people are exposed to very little rhodium in their daily lives. As it is considered to have low toxicity and may offer protective effects in some forms, no exposure limits have been set. Nevertheless, it should still be handled with care during processing.

By the way…
  • Rhodium is among the hardest and rarest elements found on Earth.
  • The name rhodium derives from the Greek word “rhodon” (rose), referring to the pink crystals observed upon its discovery.
  • Rhodium plating enhances jewellery by increasing its brightness, durability, and luster.

Created in 2026 (Project MANTRA)

Properties and applications

Periodensystem zeigt Ausschnitt mit
Platinum Group Elements @ Peter Hermes Furian – stock.adobe.com



Rhodium is a rare, shiny silver-white transition metal known for its exceptional corrosion resistance. It belongs to the platinum group elements (ruthenium, rhodium, palladium, osmium, iridium and platinum). It was discovered in 1803 by British chemist William Hyde Wollaston, who named it after the Greek word ‘rhodon’ (rose) due to the pink crystals observed during its isolation.

Rhodium is one of the hardest and rarest elements on Earth. It is durable, has high reflectivity and catalytic activity. These properties contribute to its popularity in various industrial applications. In addition to its use in alloys in catalytic converters, rhodium also finds application in a wide range of technologies .

The automotive industry uses rhodium in three-way catalytic converters. Here, it converts harmful nitrogen oxides into less harmful substances: nitrogen (N₂) and carbon dioxide (CO₂). In addition to rhodium, three-way catalytic converters often contain platinum and palladium. These are responsible for converting hydrocarbons and carbon monoxide (CO) into water and carbon dioxide (CO2) .

The chemical industry uses rhodium catalysts to produce certain basic chemicals. One example is the Ostwald process for producing nitric acid.

Rhodium is also very popular in the jewellery industry. It is extremely resistant to corrosion and oxidation, even at high temperatures. That is why it is used to refine jewellery, such as white gold or silver. This process is called ‘rhodium plating’. It ensures a bright, shiny and abrasion-resistant surface that keeps the jewellery beautiful for longer. Rhodium is very skin-friendly in jewellery (e.g. in nickel-free jewellery).

Additionally, it is utilized in the electronics industry to coat electrical contacts, preventing corrosion and enhancing conductivity. Researchers also use thin layers of rhodium for highly reflective mirrors in devices such as X-ray imaging equipment.

Rhodium is also used in nuclear reactor detectors to measure local power or neutron flux within specific reactor areas.

Rhodium also plays a specialized role in the hydrogen economy. In certain electrolysis processes, such as PEM electrolysis, it is used as a coating or alloy component to improve the efficiency of hydrogen evolution (HER – hydrogen evolution reaction). In fuel cells, rhodium alloys help prevent catalyst poisoning by carbon monoxide, extending the catalyst’s service life . Due to its high cost and limited supply, rhodium is reserved for demanding, high-performance applications.

Rhodium has no known biological function and may be harmful to health in some forms.

Occurrence and Production

Broken catalytic converters. © ToRyUK -stock.adobe.com

Rhodium is among the rarest metals on earth, with an average concentration of 0.0002 ppm. It occurs in pure form as a separate mineral and is often found alongside platinum group elements or gold.

Extracting rhodium is complex because platinum group elements are chemically similar and unreactive, making separation both difficult and costly.

As a result, recycling rhodium is increasingly important. Most recovered rhodium comes from used catalytic converters, electronic waste, and jewelry. Major producers include South Africa, Russia, and Zimbabwe, which introduces geopolitical risks .

In March 2024, platinum group elements were listed as the 27th critical material on the list of 34 strategic raw materials (SRM) under the European Critical Raw Materials Act.

 

Further information:

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Production



Rhodium is a rare precious metal belonging to the platinum group metals. It is primarily obtained together with platinum as a by-product of nickel, iron, and copper mining. As a result, specific sustainability indicators for rhodium are difficult or even impossible to determine.



The Republic of South Africa, the Russian Federation, Canada, and Zimbabwe were the world’s most important rhodium-producing countries in 2022. In 2019, global rhodium production amounted to 25 tons, of which 20 tons came from South Africa alone. Mining is carried out predominantly underground, although some operations also use open-pit methods.



Similar to ruthenium, rhodium is also a by-product of nuclear fission and can theoretically be recovered from spent nuclear fuel. However, due to the radioactivity of the other materials in the fuel elements and the associated safety requirements, rhodium is not extracted in this way.

Ressource consumption during processing

Land use for rhodium extraction is comparable to that of copper mining. Underground mining requires less surface area, as less waste rock needs to be stored.

Water consumption per ton of ore extracted is high during mining operations. In addition, energy and water consumption for further processing of the ores are substantial.


Safety

Similar to ruthenium, rhodium is non-toxic in its metallic form. In powder form, it is slightly flammable. Rhodium compounds, however, can be corrosive to the eyes, skin, and lungs, and various regulations exist for the safe handling and use of rhodium. Rhodium may be toxic to aquatic organisms. However, its toxicity is lower than that of platinum and palladium. Since it occurs only in very small quantities in the environment, no significant effects on environmental organisms are expected.

There is some evidence suggesting a potential carcinogenic effect of rhodium and its compounds.


Emissions



Rhodium is obtained as a by-product of mining other materials. Ore deposits in Russia and Canada contain very high sulfide levels. Direct sulfur dioxide emissions are therefore considered critical, and gas-cleaning systems are used during smelting to treat exhaust gases accordingly. In tailings ponds, high sulfur content can lead to acidification, which is why measures have been established to reduce sulfur levels in the waste material.



Indirect emissions generated during further ore processing must also be considered critical — primarily carbon dioxide, but also nitrogen oxides. The electrical energy used for smelting rhodium-bearing ores in South Africa (and also in Zimbabwe) is largely generated from coal-fired power plants and is therefore associated with high CO₂ emissions. Washing the ore can produce heavy metal-containing sludges that, in some mining regions, contaminate rivers and groundwater.



In contrast, rhodium is particularly important in vehicle exhaust catalysts, where it plays a key role in converting nitrogen oxides into nitrogen.

Social Aspects

The level of corporate concentration in rhodium mining is considered high. This means that only a small number of stakeholders benefit from the profits generated by its extraction.



According to the weighted country risk for rhodium-producing countries — which incorporates the World Bank’s Worldwide Governance Indicators — around 80% of production originates from countries with medium to weak governance. This indicates that in many producing countries there is a need for improvement in areas such as political stability, voice and accountability, security, and anti-corruption efforts.



For responsible metal production, supply chain due diligence within the EU is becoming increasingly important (see also the German Supply Chain Due Diligence Act). The law requires the documentation or certification of social standards and occupational health and safety measures, although initially only for companies with more than 1,000 employees.

Waste and recycling

Due to the extremely rare occurrence of rhodium, its high cost, and its versatile use as a catalyst and as a component in electrical devices, recycling is economically attractive and, for example, well established in the case of automotive catalysts. Special recovery powders are available for reclaiming rhodium from precious metal plating baths.

In addition, rhodium is present in spent nuclear fuel elements; however, it is not currently recovered from them, as the process would be highly complex due to the radioactivity of the other elements involved.


How can the material be made more sustainable?

The use of renewable energy in extraction and smelting processes could significantly reduce greenhouse gas emissions.

Almost 90% of the rhodium available in 2019 (35.5 tonnes) was used in automotive exhaust catalysts. Due to increasingly stringent emission control requirements, demand is generally expected to rise; however, the growth of electric mobility will likely reduce demand for catalytic converters.

More efficient use in catalysts, improved recovery from waste streams, and research into potential substitute materials can help reduce rhodium consumption. More sustainable use also requires high recycling rates and a circular system with minimal material losses.

Further information:


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Everyday contact

There are two main sources of exposure to rhodium (Rh) in everyday life: firstly, rhodium-plated jewelry that comes into direct contact with the skin, and secondly, car exhaust catalytic converters, which can release minimal amounts of the catalytic platinum group elements (PGE) platinum, palladium, and rhodium. When bound to fine dust in the air, these elements can also enter the lungs through respiration. A study shows that German cities can contain up to 1 mg of platinum but only 0.1 mg of rhodium per kg of dust .

Although platinum accounts for the largest proportion of PGE in road dust, the amounts of rhodium (and also palladium) excreted in human urine are higher than those of platinum, indicating better bioavailability of rhodium (and also palladium) .

 

Situation at the Workplace

In certain specialized industries (e.g., in the production of automotive catalytic converters, electroplating, or jewelry processing), workers may be exposed to rhodium, e.g., during the rhodium plating of surfaces. There is also potential for direct contact with rhodium in specialized laboratories, electronics manufacturing (e.g., rhodium-coated contacts), and the recycling industry. The main routes of exposure are inhalation of vapors, dust, or aerosols and direct contact with the skin.

Overall, exposure is relatively low. However, as some case studies show, appropriate protective measures are still required in these fields of work. A female worker in the jewelry industry developed severe skin inflammation through contact with rhodium. A 27-year-old worker suffered mild asthma attacks and inflammation of the nasal mucosa at a rhodium surface coating plant. These are rare isolated cases. Nevertheless, sensitization to rhodium compounds cannot be ruled out, as is also the case with other PGE zotpressInText item="{4274171:XIITX6U4},{4274171:NRCGLCMM}"] (see platinum, palladium, and ruthenium).

There are no fixed occupational exposure limits for rhodium in Germany, Europe, or the USA. For some rhodium compounds, there are guidelines for possible skin contact, but for rhodium-containing dusts, only the general dust limit must be observed (see Granular biopersistent dust particles).

 

Products and Customer

Private individuals rarely come into contact with rhodium. Rhodium is hardly toxic in metallic form. It is mainly used in products such as jewelry (especially silver and white gold jewelry, known as rhodium-plated jewelry) and car catalytic converters. Rhodium-plated jewelry is considered hypoallergenic and harmless. However, rhodium can cause allergic reactions in a few very sensitive individuals.

Environmental pollution caused by rhodium (e.g., from car catalytic converters) has been proven, but it is minimal and poses no health risk . Rhodium is not used in cosmetics or food. Overall, rhodium does not pose a relevant health risk to consumers.

 

Under normal conditions, consumers face a very low risk of exposure to rhodium. Rhodium-plated jewelry is generally safe. Environmental pollution from rhodium is low. However, people with known metal allergies should be aware of possible reactions to rhodium salts and, if necessary, avoid wearing rhodium-plated jewelry. The World Health Organization (WHO), the Agency for Toxic Substances and Disease Registry (ATSDR) in the USA, the European Chemicals Agency (ECHA), and other authorities do not currently classify rhodium as relevant for widespread consumer exposure.

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Rhodium (Rh) enters the environment mainly through the abrasion of automotive catalytic converters, usually in very small amounts. It mostly occurs as fine dust and is difficult to detect in many samples. Laboratory studies show that rhodium remains chemically stable and can be more soluble under certain conditions. Overall, rhodium is released continuously, but in much smaller amounts than other similar metals.


Release



Rhodium is introduced into the environment almost exclusively through human activities. Since the 1980s, road traffic has been the dominant source. Rhodium is used in three-way catalytic converters for exhaust gas treatment

























. Its release mainly occurs through abrasion of catalyst surfaces and the detachment of catalytic support materials, which enter the exhaust stream as fine metallic and oxide rhodium particles (Bluteau et al. 2025; Colombo et al. 02-2008).



The amount emitted depends on driving behaviour, speed, the age and condition of the catalytic converter, and the engine type. Particularly high rhodium releases occur during cold starts, under high engine load, and from older petrol vehicles whose catalytic converter systems operate less efficiently. Diesel vehicles emit rhodium mainly during urban cold-start phases, before the catalyst has reached its operating temperature

























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Traffic-related abrasion leads to an initial accumulation of rhodium in road dust and on surfaces along traffic routes

























. Rainfall and surface runoff mobilise rhodium from road environments and transport it further into urban drainage systems. These systems represent an important secondary transport pathway

























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In addition to traffic, mining, metal production, industrial processes, and waste and sewage sludge treatment also contribute to the release of small amounts of rhodium. On a global scale, however, these sources play a clearly subordinate role

























. In urban rivers and streams, most rhodium input is attributed to traffic-related sources

























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Monitoring programmes show that rhodium is often detected in environmental samples only at very low concentrations or below the detection limit. This indicates that its release rate is low compared with other platinum group elements

























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Global trend analyses further indicate a substantial rise in rhodium emissions driven by the increasing use of vehicle catalytic converters. As a result, rhodium is now frequently detected in soils, road dust, and runoff-affected environments (Alfakhory et al. 2021; Colombo et al. 08-2008).


Released amounts


Rhodium occurs in the environment almost exclusively at very low concentrations, with the highest values consistently measured in compartments close to roads.



In air, rhodium concentrations are typically in the pg/m³ range. Highly polluted traffic sites reach about 1–4 pg/m³, rarely up to 12 pg/m³, while low-traffic sites show concentrations of 0.5–1 pg/m³. PM2.5 usually contains about 1–1.6 pg/m³ rhodium, making rhodium the least abundant platinum group element in outdoor air

























. According to available studies, the maximum daily inhalation dose is about 0.004 ng rhodium/m³.



Global releases from road traffic are estimated at around 3 tonnes of rhodium per year. Local soil contamination varies strongly: background areas contain less than 0.1 mg/kg, whereas heavily trafficked roads can reach up to 10–20 mg/kg. Concentrations of up to 22 mg/kg have been detected in sewage sludge

























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Measurements of Euro 3–5 vehicle systems for petrol and diesel engines show low emission rates in the range of a few nanograms per analysed filter. Euro 3 petrol vehicles emit about 2.1–5.1 ng, while newer standards, Euro 4 and Euro 5, are mostly close to the detection limit of the analytical system, which is 0.005 µg/L

























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In road dust, rhodium concentrations have mostly been reported in the range of several to several hundred ng/g, typically around 10–200 ng/g. In Montreal, for example, 7.1 ng/g was measured in road dust and 1.6 ng/g in sediments (Lesniewska et al. 2004; Bluteau et al. 2025). Tunnel dust contains slightly lower concentrations, while grass samples contain only very small or non-detectable amounts

























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Rhodium can also be measured in urban waters, but only at very low concentrations. In sediments, values occur in the lower ng/g range; in water samples, concentrations are usually below the detection limit or only around 0.05–0.3 ng/L (Moldovan et al. 2001; Egorova et al. 2019; Colombo et al. 08-2008).

Results from the laboratory on release

Laboratory analyses have shown that rhodium is first present in metallic form. During the use of catalytic converters, however, it can be oxidised. As a result, more soluble rhodium can be released from aged catalytic converters. The share of soluble rhodium clearly increases when the catalyst is physically or chemically aged. The pH value also affects how much rhodium is released

























. Quantitative analyses show clear differences between new and strongly aged catalytic converters: modern systems contain high and stable rhodium levels, while old catalytic converters can release much larger amounts

























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Laboratory experiments also show that rhodium is only slightly soluble. However, it can be mobilised from road dust under different pH conditions. In neutral solutions, only very small amounts are released. Under acidic conditions, the release is much higher. This suggests that rhodium can occur in different chemical forms and become more mobile (Moldovan et al. 2001; Colombo et al. 02-2008). In further tests, rhodium chloride (RhCl₃) remained completely soluble and stable. It did not hydrolyse or precipitate. This shows that dissolved rhodium compounds can be very persistent under laboratory conditions (Colombo et al. 08-2008).

Dissolved rhodium species were found only at very low concentrations in solubility tests. This suggests that rhodium is generally released only in small amounts, or that it often occurs in stable and poorly mobile forms

























. Finally, laboratory studies and analyses of treatment plants show that rhodium is not lost during thermal processes. Instead, it accumulates in the ash

























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Rhodium is regularly released into the environment through road traffic, but it is usually only present in trace amounts. It is stable, but under certain conditions it can become quite soluble. For this reason, it should continue to be monitored, even though current levels are still low.

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Rhodium can be absorbed through both the lungs and the skin, particularly in its ionic form (as rhodium salts). However, the amounts are very small. Significant exposure only occurs in specific workplaces.

 

Uptake via the Lung


The uptake of rhodium (Rh) through the lungs is caused almost exclusively by car exhaust catalysts. A German study showed that people in cities had more rhodium in their urine than people in rural areas. The differences were very small. Although the amounts of rhodium absorbed are higher than those of platinum (Pt), they are well below levels of concern. Similar results had previously been found in tram drivers in Rome. They showed slightly elevated levels of platinum and rhodium in their urine compared to a control group. An Indian study also showed a correlation: traffic police officers in Hyderabad were tested for blood levels of platinum, palladium, and rhodium depending on their length of service . All three elements were elevated in the blood of the individuals examined compared to the unexposed control group. Further studies show that artificial lung fluid is capable of dissolving rhodium and other platinum group metals from road dust – which explains the exposure in the body .

 

Uptake via the skin

The few studies that have investigated the absorption of rhodium through the skin have used a special in vitro system known as the Franz diffusion cell. In this system, real skin (e.g., pig skin or skin samples from volunteers) is clamped into a vessel in such a way that the “outside” and “inside” are spatially separated from each other. Although this experimental setup is prone to errors (e.g., due to leaks on the clamped sides of the skin sample), it can replace many animal experiments.

When skin in the Franz diffusion cell was exposed to soluble platinum or rhodium salts, significantly more platinum was transported through the skin or deposited in the skin than rhodium salts. However, after 24 hours, the amounts that had passed through the skin were still very small. The results were quite different when platinum and rhodium nanoparticles were examined. Although these particles were very small (diameter < 10 nm), no transport through healthy skin could be detected. Only when the skin was injured with a needle could the penetration of particles from “outside” to “inside” be measured in the Franz diffusion cell. In a third study, a soluble rhodium salt (rhodium trichloride, RhCl3) was applied to the skin at different pH values and its penetration through the skin was analysed. The study was able to demonstrate that acidic conditions (low pH) led to an increase in the amount of rhodium transported through the skin .

 

Uptake via the gastrointestinal Tract


In everyday life, platinum group elements (PGEs) play hardly any role in the gastrointestinal tract, as rhodium is not absorbed through food or drinking water. Nevertheless, a study investigated whether platinum, palladium, or rhodium particles released from automotive exhaust catalysts would be bioavailable at all if they entered the stomach and intestines . For this purpose, artificial stomach and intestinal fluid was used to investigate how much platinum, palladium, and rhodium could be dissolved from particulate matter. Palladium and rhodium were more soluble than platinum. Nevertheless, the available amount was highest for platinum because it occurs most frequently in catalytic converters. However, since very little is absorbed through the gastrointestinal tract, this route plays hardly any role in health .

 

Uptake via medical applications


Like other platinum group elements, rhodium could also play a role as a potential tumor drug. Scientists use rhodium in compounds that force their way into the DNA of tumor cells (technical term: “intercalation”). As so-called “metalloinsertors,” these compounds prevent tumor cells from dividing. Another application is the combination of rhodium citrate with so-called SPIONs, small magnetic iron nanoparticles. These can be directed to the tumor site from outside using a magnetic field, thus delivering the effective rhodium citrate to the tumor cells [16-18]. Both applications are still in the research stage and have not yet reached clinical application .

 

Rhodium intake is mainly seen in occupational settings (employees in transport or industry). For the general population, all findings to date show that rhodium is hardly absorbed and poses no health risk.

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Rhodium causes only minor acute effects in environmental organisms. Even high concentrations rarely lead to strong responses in laboratory experiments. However, certain chemical forms of rhodium, especially soluble rhodium(III) complexes, can become biologically active. Actual environmental concentrations are far below critical thresholds.


Uptake


Rhodium is present in the environment only in very small amounts. In environmental samples, rhodium is found almost exclusively in filter-feeding mussels. Fish, plants, and humans show hardly any measurable amounts

























. Since rhodium mainly occurs as particles in dust, inhalation is the main and therefore most important exposure and risk pathway for air-breathing organisms (Wiseman 2009).



Plants can take up rhodium both in dissolved form, for example as salts, and in undissolved form as nanoparticles. Accumulation mainly occurs in the roots. Because rhodium concentrations in soil are low, soil invertebrates such as earthworms or woodlice take up only very small amounts of rhodium (Schertzinger et al. 2017). Fish and mussels also take up only small amounts of rhodium through water or food. Most of the absorbed rhodium is excreted again within a short time (Bluteau et al. 2025). Uptake mainly occurs via sediment-bound particles

























. Compared with other platinum group elements, only small amounts are taken up.



Plants growing close to roads show traces of rhodium as deposits on leaf surfaces. This is not systemic uptake. In trees, higher plant parts remain largely uncontaminated, which reflects the very local release of rhodium from traffic-related sources

























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Highly soluble rhodium salts, which are used as starting materials in catalyst chemistry, can be taken up quickly and in large amounts by plants under laboratory conditions. However, this is not a typical environmental pathway (Colombo et al. 08-2008). Animal studies confirm that rhodium can in principle be taken up through the digestive tract, distributed in the body, and excreted via the kidneys. This shows that biological uptake is possible, even though environmental availability remains very low overall (Egorova et al. 2019; Lavicoli et al. 2014). Exposure studies also show only moderate acute effects, and bioaccumulation remains low. This further supports the limited toxic impact of rhodium (Moldovan et al. 2001; Wiseman 2009).

Toxicity



Rhodium generally shows very low toxicity to environmental organisms. Even high concentrations cause only minor effects. Rhodium is clearly less toxic than comparable platinum group elements. In freshwater, it is considered to have low toxicity (Zimmermann et al. 2015), and it does not affect the growth, reproduction, or development of organisms (Schertzinger et al. 2017).



Despite its low environmental toxicity, some laboratory studies show that rhodium can cause genotoxic or oxidative effects in vitro, although these effects are much weaker than those caused by platinum.



Toxic effects have been observed for several soluble rhodium(III) complexes. These complexes are mobile and bioavailable. Plants only respond at high rhodium(III) concentrations, while animals require even higher, extreme amounts before effects occur (Colombo et al. 02-2008; Colombo et al. 08-2008). In animal models, dose-dependent indications of damage to kidney tubules were found. This shows that rhodium can cause specific organ damage at high concentrations (Egorova et al. 2019; Lavicoli et al. 2014).



In aquatic invertebrates, rhodium concentrations of 3.6–48 ng/g were measured depending on the sampling site. This confirms a widespread, but very low, traffic-related input of rhodium into the environment

























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Assessment of the Effects of Material XY on Environmental Organisms


In experiments with soil and aquatic organisms, no effects on growth, reproduction, or fertility were observed, even at extremely high concentrations. Rhodium is therefore classified as the least toxic element within the platinum group elements, and population-level effects are not expected under environmental conditions (Schertzinger et al. 2017).Since environmental concentrations are low and only very few effects occur in environmental organisms, rhodium currently does not pose an acute environmental risk. However, chronic effects in daphnids cannot be completely ruled out after long-term exposure or under changed chemical conditions (Bluteau et al. 2025). Although the rhodium salt RhCl₃ shows strong phytotoxicity in th e laboratory, environmental concentrations are several orders of magnitude lower. Therefore, the acute risk for plants is low (Colombo et al. 08-2008).

Rhodium currently shows no relevant acute toxicity to environmental organisms. Acute risks are very low because typical environmental concentrations are far below toxic levels. Only specific rhodium compounds may show stronger effects in individual cases. Overall, the ecological risk is currently considered low.

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Metallic rhodium has virtually no harmful effects on the body. Large quantities of rhodium salt may have some effects on the body under certain circumstances. However, even road traffic dust and exhaust fumes do not contain quantities of this magnitude.

 

Behaviour inside the Body


In a scientific study, researchers treated rats with a rhodium salt (rhodium-(III)-chloride) in their drinking water for two weeks. Dosages of 0.25 mg, 0.5 mg, and 1 mg of rhodium salt per liter led to mild kidney dysfunction. However, the effects varied greatly and have not yet been confirmed .

A clinical study reports on a 27-year-old male patient who was observed to have rhodium sensitization .. The man had worked for three years with electrolytic baths for plating, during which time he came into contact with platinum-, gold-, and mainly rhodium salts. According to this study, rhodium can, in very rare cases, have a sensitizing or allergenic effect on humans.

However, the body of research available for assessing the toxicity of rhodium in humans remains very limited. This is also due to the fact that this metal has a very low effect when it interacts with biological systems. For this reason, the International Agency for Research on Cancer (IARC) has not classified rhodium as a possible carcinogen. However, both the Health Council of the Netherlands in 2002 and the German MAK Commission (2023) point out that there are too few studies on rhodium salts and that these compounds cannot be evaluated with sufficient certainty. The Health Council of the Netherlands therefore called for classification in the former EU category 3B (possibly carcinogenic), which currently corresponds to category 2 following the redefinition of classifications in the EU in 2008 (EUR-Lex No. 1272/2008, Annex 1, 3.6). The MAK Commission has also classified rhodium and its compounds as possibly carcinogenic, which corresponds to KanzKat 3 in the special regulations in Germany. In all cases, however, the classification was made due to a lack of studies and not because of existing evidence. Therefore, these classifications are currently to be seen as a safety measure in accordance with the precautionary principle and not as a scientifically justified measure.

 

Uptake and Effects in Cells


Most data on rhodium comes from experiments with cell cultures. Comparisons with other platinum group elements (PGEs) in particular allow some important conclusions to be drawn about rhodium.

Several studies have shown that rhodium is the element with the weakest biological effect (compared to platinum and palladium). In various cell tests, platinum was 30 times more effective than rhodium and palladium was 3 times more effective [20]. Another study using human lung cells confirmed the same order and the same differences in toxicity .

Rhodium nanoparticles (15 nm) have even been observed to reduce the formation of oxygen radicals, thereby inhibiting lipid peroxidation and protecting human intestinal cells (Caco-2) from the harmful effects of hydrogen peroxide (H2O2) without themselves causing any toxic effects .

However, the situation is different when soluble salts are examined. Rhodium salts inhibit the growth, division, and viability of various cultured cells .

When in contact with freshly isolated human lymphocytes, platinum, palladium, and rhodium salts can negatively affect cell growth and the formation of messenger molecules (cytokines) . Here, too, rhodium is the least effective and requires very high concentrations to have an effect.

 

Rhodium is very low in toxicity and, as a metallic nanoparticle, even has a protective effect. For this reason, there are no specific limit values for this element.

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Transport

Rhodium changes only slowly after it is released into the environment. It often binds to particles. However, under certain chemical conditions, it can change into more soluble forms. Fine dust, road dust, and sediments play the most important role in this process. Dissolved forms of rhodium are rare, but they can strongly increase its mobility. Overall, the transport of rhodium strongly depends on its chemical form.




Rhodium shows high chemical stability and only low binding to surfaces under neutral conditions (Zimmermann et al. 2015). In the environment, rhodium is mainly transported via atmospheric particles, road dust, and sediment redistribution. Very fine fractions below 10 µm are especially important for local and regional transport (Wiseman 2009). Long-range transport through the atmosphere and deposition in snow has also been shown. In soils, rhodium is not very mobile because it binds to minerals and organic matter

























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The main transport pathway starts with road abrasion, followed by surface runoff into water bodies. There, rhodium quickly binds to sediments and shows only limited mobility in dissolved form (Omrani et al. 2020; Moldovan et al. 2001). Fine road and tunnel dust can contain locally elevated amounts, but most of this material remains close to the emission sources

























. Because environmental concentrations are very low, rhodium is difficult to detect in water. Its mobility is likely comparable to that of palladium (Alfakhory et al. 2021).



Laboratory and field analyses show that rhodium in urban environments is mainly transported in particle-bound form and is redistributed through sewer systems, rivers, and sediments, while dissolved forms occur only rarely (Bluteau et al. 2025). Chlorinated rhodium(III) complexes can be transported in aquatic systems (Colombo et al. 02-2008). Dissolved rhodium(III) species show high stability in water and can be detected in wastewater treatment plants (Colombo et al. 08-2008; Jackson et al. 2010).

Transformation


In the environment, rhodium remains chemically unchanged under neutral conditions: it does not precipitate, does not form hydroxides, and shows no loss in concentration (Zimmermann et al. 2015). Under acidic or basic conditions, however, rhodium can change into more soluble and therefore more bioavailable forms through oxidation, chlorination, or binding to organic substances (Wiseman 2009). Such transformations have already been detected in road dust (Bluteau et al. 2025).



Its low toxicity can be explained by the fact that rhodium interacts only weakly with biochemically important molecules. It binds only weakly to sulphur-containing groups in molecules and does not activate metal-binding proteins, such as metallothioneins (Schertzinger et al. 2017 ). After release from catalytic


converters, rhodium slowly changes from metallic rhodium⁰ to oxidised rhodium(III) species. Depending on the pH, these species can form hydroxide or organic complexes. However, they usually remain bound to solids and show overall low mobility

























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Chloride-, nitrate-, and humic substance-bound Rh(III) complexes have high solubility, while rhodium oxides or metallic particles are highly stable and accumulate locally (Colombo et al. 02-2008). In natural waters, the form in which rhodium occurs depends on the speed of chemical transformation and the formation of Rh complexes. Rhodium may therefore occur either as a dissolved and mobile form or as a strongly bound and less reactive form. This helps explain why rhodium is generally very difficult to detect (Egorova et al. 2019; Alfakhory et al. 2021)
Rhodium is usually stable and remains mainly bound to solids, but it can be mobilised under certain conditions. Its transport occurs mainly through fine particulate matter, road runoff, and sediments. Dissolved forms are rare, but they can potentially be transported more easily. Overall, rhodium shows only limited environmental spread.

Created in 2026 (Project MANTRA)


1.
Bystrzanowska, M.; Petkov, P.; Tobiszewski, M. Ranking of Heterogeneous Catalysts Metals by Their Greenness. Acs Sustain Chem Eng 2019, 7, 18434–18443, https://doi.org/10.1021/acssuschemeng.9b04230.
1.
Zereini, F.; Wiseman, C.; Alt, F.; Messerschmidt, J.; Muller, J.; Urban, H. Platinum and Rhodium Concentrations in Airborne Particulate Matter in Germany from 1988 to 1998. Environmental science & technology 2001, 35, 1996–2000, https://doi.org/10.1021/es001126z.
1.
Zereini, F.; Wiseman, C.L.; Puttmann, W. In Vitro Investigations of Platinum, Palladium, and Rhodium Mobility in Urban Airborne Particulate Matter (PM10, PM2.5, and PM1) Using Simulated Lung Fluids. Environmental science & technology 2012, 46, 10326–10333, https://doi.org/10.1021/es3020887.
1.
Schmid, M.; Zimmermann, S.; Krug, H.F.; Sures, B. Influence of Platinum, Palladium and Rhodium as Compared with Cadmium, Nickel and Chromium on Cell Viability and Oxidative Stress in Human Bronchial Epithelial Cells. Environment international 2007, 33, 385–390, https://doi.org/10.1016/j.envint.2006.12.003.
1.
Munker, S.; Kilo, S.; Ross, C.; Jeitner, P.; Schierl, R.; Goen, T.; Drexler, H. Exposure of the German General Population to Platinum and Rhodium – Urinary Levels and Determining Factors. International journal of hygiene and environmental health 2016, 219, 801–810, https://doi.org/10.1016/j.ijheh.2016.07.019.
1.
Merget, R.; Sander, I.; van Kampen, V.; Raulf-Heimsoth, M.; Ulmer, H.M.; Kulzer, R.; Bruening, T. Occupational Immediate-Type Asthma and Rhinitis Due to Rhodium Salts. Am J Ind Med 2010, 53, 42–46, https://doi.org/10.1002/ajim.20786.
1.
Kamala, C.T.; Balaram, V.; Satyanarayanan, M.; Kiran Kumar, A.; Subramanyam, K.S. Biomonitoring of Airborne Platinum Group Elements in Urban Traffic Police Officers. Arch Environ Contam Toxicol 2015, 68, 421–431, https://doi.org/10.1007/s00244-014-0114-7.
1.
Mauro, M.; Crosera, M.; Bianco, C.; Adami, G.; Montini, T.; Fornasiero, P.; Jaganjac, M.; Bovenzi, M.; Filon, F.L. Permeation of Platinum and Rhodium Nanoparticles through Intact and Damaged Human Skin. Journal of Nanoparticle Research 2015, 17, https://doi.org/ARTN 253 10.1007/s11051-015-3052-z.
1.
Jansen Van Rensburg, S.; Franken, A.; Du Plessis, J.; Du Plessis, J.L. The Influence of pH on the in Vitro Permeation of Rhodium through Human Skin. Toxicol Ind Health 2017, 33, 487–494, https://doi.org/10.1177/0748233716675218.
1.
Iavicoli, I.; Cufino, V.; Corbi, M.; Goracci, M.; Caredda, E.; Cittadini, A.; Bergamaschi, A.; Sgambato, A. Rhodium and Iridium Salts Inhibit Proliferation and Induce DNA Damage in Rat Fibroblasts in Vitro. Toxicology in vitro : an international journal published in association with BIBRA 2012, 26, 963–969, https://doi.org/10.1016/j.tiv.2012.03.014.
1.
Iavicoli, I.; Bocca, B.; Carelli, G.; Caroli, S.; Caimi, S.; Alimonti, A.; Fontana, L. Biomonitoring of Tram Drivers Exposed to Airborne Platinum, Rhodium and Palladium. International archives of occupational and environmental health 2007, 81, 109–114, https://doi.org/10.1007/s00420-007-0195-y.
1.
Franken, A.; Eloff, F.C.; Du Plessis, J.; Badenhorst, C.J.; Jordaan, A.; Du Plessis, J.L. In Vitro Permeation of Platinum and Rhodium through Caucasian Skin. Toxicology in vitro : an international journal published in association with BIBRA 2014, 28, 1396–1401, https://doi.org/10.1016/j.tiv.2014.07.007.
1.
Colombo, C.; Monhemius, A.J.; Plant, J.A. The Estimation of the Bioavailabilities of Platinum, Palladium and Rhodium in Vehicle Exhaust Catalysts and Road Dusts Using a Physiologically Based Extraction Test. The Science of the total environment 2008, 389, 46–51, https://doi.org/10.1016/j.scitotenv.2007.08.022.
1.
Colombo, C.; Monhemius, A.J.; Plant, J.A. Platinum, Palladium and Rhodium Release from Vehicle Exhaust Catalysts and Road Dust Exposed to Simulated Lung Fluids. Ecotox Environ Safe 2008, 71, 722–730, https://doi.org/10.1016/j.ecoenv.2007.11.011.
1.
Carneiro, M.L.B.; Lopes, C.A.P.; Miranda-Vilela, A.L.; Joanitti, G.A.; da Silva, I.C.R.; Mortari, M.R.; de Souza, A.R.; Bao, S.N. Acute and Subchronic Toxicity of the Antitumor Agent Rhodium (II) Citrate in Balb/c Mice after Intraperitoneal Administration. Toxicol Rep 2015, 2, 1086–1100, https://doi.org/10.1016/j.toxrep.2015.07.010.
1.
Carneiro, M.L.; Peixoto, R.C.; Joanitti, G.A.; Oliveira, R.G.; Telles, L.A.; Miranda-Vilela, A.L.; Bocca, A.L.; Vianna, L.M.; da Silva, I.C.; de Souza, A.R.; et al. Antitumor Effect and Toxicity of Free Rhodium (II) Citrate and Rhodium (II) Citrate-Loaded Maghemite Nanoparticles in Mice Bearing Breast Cancer. Journal of nanobiotechnology 2013, 11, 4, https://doi.org/10.1186/1477-3155-11-4.
1.
Carneiro, M.L.; Nunes, E.S.; Peixoto, R.C.; Oliveira, R.G.; Lourenco, L.H.; da Silva, I.C.; Simioni, A.R.; Tedesco, A.C.; de Souza, A.R.; Lacava, Z.G.; et al. Free Rhodium (II) Citrate and Rhodium (II) Citrate Magnetic Carriers as Potential Strategies for Breast Cancer Therapy. Journal of nanobiotechnology 2011, 9, 11, https://doi.org/10.1186/1477-3155-9-11.
1.
Bailis, J.M.; Weidmann, A.G.; Mariano, N.F.; Barton, J.K. Rhodium Metalloinsertor Binding Generates a Lesion with Selective Cytotoxicity for Mismatch Repair-Deficient Cells. Proc Natl Acad Sci U S A 2017, 114, 6948–6953, https://doi.org/10.1073/pnas.1706665114.
1.
Cao, G.J.; Chen, Y.; Chen, X.; Weng, P.; Lin, R.G. Intrinsic Catalytic Activity of Rhodium Nanoparticles with Respect to Reactive Oxygen Species Scavenging: Implication for Diminishing Cytotoxicity. J Environ Sci Health C Environ Carcinog Ecotoxicol Rev 2019, 37, 14–25, https://doi.org/10.1080/10590501.2019.1555319.
1.
Bunger, J.; Stork, J.; Stalder, K. Cyto- and Genotoxic Effects of Coordination Complexes of Platinum, Palladium and Rhodium in Vitro. International archives of occupational and environmental health 1996, 69, 33–38, https://doi.org/10.1007/BF02630736.
1.
Bocca, B.; Alimonti, A.; Cristaudo, A.; Cristallini, E.; Petrucci, F.; Caroli, S. Monitoring of the Exposure to Platinum-Group Elements for Two Italian Population Groups through Urine Analysis. Anal Chim Acta 2004, 512, 19–25, https://doi.org/10.1016/j.aca.2004.02.032.
1.
Boscolo, P.; Di Giampaolo, L.; Reale, M.; Castellani, M.L.; Ritavolpe, A.; Carmignani, M.; Ponti, J.; Paganelli, R.; Sabbioni, E.; Conti, P.; et al. Different Effects of Platinum, Palladium, and Rhodium Salts on Lymphocyte Proliferation and Cytokine Release. Ann Clin Lab Sci 2004, 34, 299–306.
1.
Housecroft, C.E.; Sharpe, A.G. Inorganic Chemistry; second.; Pearson Education Limited: Harlow, Essex CM20 2EJ, England, 2005; ISBN 0-13-039913-2.
1.
Yentekakis, I.V.; Konsolakis, M. Three‐Way Catalysis. In Perovskites and Related Mixed Oxides; Granger, P., Parvulescu, V.I., Prellier, W., Eds.; Wiley-VCH: Weinheim, Germany, 2016; pp. 559–586 ISBN 978-3-527-33763-7 978-3-527-68660-5.
1.
Schmidt, M. Rohstoffrisikobewertung – Platingruppenmetalle; Deutsche Rohstoffagentur (DERA), Ed.; Platin, Palladium, Rhodium; Bundesanstalt für Geowissenschaften und Rohstoffe, 2014; ISBN 978-3-943566-19-2.
1.
Pérez, G.; Díaz-Sainz, G.; Gómez-Coma, L.; Álvarez-Miguel, L.; Garnier, A.; Cabon, N.; Ortiz, A.; Gloaguen, F.; Ortiz, I. Rhodium-Based Cathodes with Ultra-Low Metal Loading to Increase the Sustainability in the Hydrogen Evolution Reaction. Journal of Environmental Chemical Engineering 2022, 10, 107682, https://doi.org/10.1016/j.jece.2022.107682.
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