What is chromium (VI)?

Chromium is a metal that is found in various states of oxidation, such as chromium (III) and chromium (VI), also known as trivalent and hexavalent chromium respectively. Chromium (VI) is always formed as part of another substance and cannot exist on its own. After initially binding with oxygen, for example, chromium (VI) is primarily found as chromate and dichromate. Chromate and dichromate ions are always associated with other substances such as zinc or sodium. We therefore always refer to ‘chromium (VI) compounds’. 

Chromium (VI) compounds are generally made by humans for their rust-resistant properties. Chromium (VI) occurs primarily in the form of minerals and salts. Every chromium (VI) compound has its own colour, specific composition and unique properties, varying in terms of particle size and their solubility in water, for instance.

How can you come into contact with chromium (VI)?

We can all be exposed to small quantities of chromium (VI) in our daily lives. However, the exposure in work environments can be many times higher than elsewhere.

Sources of chromium (VI) in the living environment 

The principal application of chromium (VI) is in rust-proofing paint. It is also sometimes used in preserved wood (e.g. for play equipment, fences, and wooden garden structures). Exposure can occur when preserved wood is processed or burned. However, in practice, this exposure is so low that the associated health risks are very small. Chromium (VI) can also be released when welding stainless steel or when processing chrome-plated products such as taps. It can also occur in leather (as an impurity) and in cement. For these and for other consumer products such as cosmetics, cleaning agents, toys and tobacco, legal limits have been set and the health risks due to exposure are negligible. Foodstuffs do not contain chromium (VI) as this is converted into harmless chromium (III) before consumption. Drinking water in the Netherlands contains hardly any or no chromium (VI). A standard has been defined in the Netherlands for the total amount of chromium in drinking water; there is no separate standard for chromium (VI). 

The background levels measured in the air outdoors in the Netherlands are lower than the maximum permissible concentration for chromium (VI) of 2.5 ng/m3 (based on the risk level for carcinogenic substances). In the vicinity of industrial sources (e.g. wood preserving companies, iron foundries and chrome smelters), higher concentrations in the air have occasionally been measured (up to 19 ng/m3). This does not, however, result in  an unacceptable risk for the surrounding areas.

Sources of chromium (VI) at the workplace

Exposure to chromium (VI) at the workplace is possible in the following situations (amongst others):

• When spraying paint that contains chromium (VI)
• When sanding, welding, cutting or sawing surfaces which have a coating of paint that contains chromium (VI)
• When welding stainless steel
• When chrome-plating metal or plastic surfaces, which involves adding chromium (VI) compounds and reducing these to chromium (0) on the metal or plastic
• When preserving wood or processing preserved wood
• When working with cement
• During work in or near a tannery, or when processing leather

The standard set for air borne chromium (VI) in the workplace is higher than it is for the outside air. This is because exposure at the workplace is for a shorter period (not 24 hours a day, 7 days a week, lifelong). Readily soluble chromium (VI) compounds are taken up into the body more easily than poorly soluble compounds. For that reason, the legal limit for the workplace is higher for poorly soluble chromium (VI) compounds. 

The duration, level and exposure route differ depending on the working situation. The type of chromium (VI) compound also affects health risk, as do the working conditions, such as the use of personal protection equipment, for example.

It is possible to work safely with chromium (VI) and clean areas where work with chromium (VI) has been done. It is important to wear Personal Protective Equipment (PPE) such as breathing masks, gloves and workwear. Proper ventilation in the working areas that is suitable for the particle size is also important.

Exposure routes to chromium (VI)

When paint containing chromium (VI) has dried, the chromium (VI) is fixed in the paint and there is no longer any exposure. Processing paint that contains chromium (VI), for instance by sanding or welding , releases particles that contain chromium (VI). Once released, there are three ways that chromium (VI) can be absorbed into the body: by inhalation, by swallowing, and by penetrating the skin. The exposure route determines which health effects can occur. 

All three pathways require the substance to pass through lipophilic cell membranes. Chromium (VI), in the form of a chromate anion, can easily pass through cell membranes via the phosphate and sulphate anion channels, even though it is not an essential substance for the cell. Dichromate anions are hydrophilic, they exist in equilibrium with chromate anions in aqueous environments which are available for uptake into the cell.

Uptake via the lungs

Chromium (VI) compounds are non-volatile and therefore do not appear as a gas or vapour. They can, however, be inhaled via dust particles (released e.g. by sanding objects coated with paint that contains chromium) and in liquid aerosols (created e.g. by spraying the paint). The smaller the particles, the deeper they can penetrate into the lungs. Larger particles may be filtered by the nasal hairs or can penetrate less deeply into the respiratory tract. The solubility of the compounds in water also determines where the effects of exposure will occur. Compounds that are more highly soluble can have an effect higher up the respiratory tract (e.g. in the nasal septum and the nasal mucosa) than poorly soluble compounds, which end up lower down the respiratory tract.

Uptake via the gastrointestinal tract 

Chromium (VI) compounds can be swallowed as solid particles or may be dissolved in water. It is also possible to swallow particles or aerosols that have been inhaled and then coughed up. After they have been swallowed, the particles will be dissolved or broken down in the stomach and the chromium (VI) released. It can then be dissolved in the gastric juices and absorbed by the intestines. The particle size and how easily the particles break down in the gastric acids are determining factors for this process. Some of the chromium (VI) will be converted into chromium (III) while still in the stomach, thereby lowering the amount that reaches the intestines.

Uptake via the skin

Chromium (VI) can pass the epidermis and reach the underlying dermis. It can then be absorbed into the body through the blood vessels in the dermis.

Conversion of chromium (VI) to chromium (III)

From the first moment of contact onwards, chromium (VI) is reduced to the less toxic chromium (III). The conversion starts at first contact of the body with chromium (VI). Ascorbic acid (vitamin C) and glutathione (GSH) play an important role in this reduction. Lower concentrations of these reducing agents in the body result in a higher absorption of chromium (VI). In work environments, the conversion of chromium (VI) depends on a number of conditions, such as the form in which it is released and the amount of moisture it comes into contact with. This will determine the extent to which chromium (VI) is dissolved and consequently the absorption of chromium (VI).

Most of the chromium (VI) is converted into chromium (III) before it reaches the bloodstream. The chromium (VI) that does get into the blood enters red blood cells where it is subsequently reduced to chromium (III). 

Conversion outside the body cells

The conversion of chromium (VI) to chromium (III) outside the body cells, for example in gastric juices, pulmonary mucosa or sweat, , is not harmful to human health. For the most part, chromium (VI) compounds converted to chromium (III) before absorbtion into the body. However, the greater the amount of chromium (VI) compounds that ends up in gastric juices, pulmonary mucosa or sweat, the more will remain to penetrate the body cells and cause damage.

Conversion inside a body cell

If the conversion of chromium (VI) to chromium (III) takes place inside a body cell, it can cause harm to that cell. During conversion, free oxygen radicals that can damage the cell are produced and at sites with damaged cells, health problems can arise. The greatest risk for developing health problems is therefore in the tissues that are close to the point of initial contact. Besides that, the susceptibility of the tissue is also a contributing factor. 

Chromium (III) is excreted in the faeces or urine. If the chromium (III) has been taken up into a blood cell, it will be excreted when that blood cell is broken down (after up to about 120 days).

Chromium (VI) and diseases

Scientific knowledge states that occupational exposure to chromium (VI) can cause various illnesses. 

Chromium (VI) can cause the following irreversible adverse health effects in humans:

• Lung cancer
• Nose and nasal sinus cancer
• Nasal septum perforation caused by chromium ulcers
• Chronic lung diseases, e.g. COPD, pulmonary fibrosis and irritant-induced asthma
• Allergic asthma
• Allergic rhinitis
• Allergic contact dermatitis

Chromium (VI) is suspected to cause the following irreversible adverse health effects in humans:

• Stomach cancer
• Laryngeal cancer

There is no or insufficient evidence as to whether chromium (VI) can cause the following conditions in humans:

• Effects on reproduction and prenatal development
• Immune-mediated diseases, other than those mentioned before
• Cancer of the small intestine 
• Cancer of the oral cavity
• Pancreatic cancer
• Prostate cancer
• Bladder cancer

Little or no convincing indications has been found in studies of humans and animals that chromium (VI) can cause the following conditions:

• Dental effects
• Types of cancer other than those mentioned above
• Disorders of the gastrointestinal tract
• Adverse health effects on the haematological system
• Liver diseases
• Kidney diseases
• Central nervous system disorders
• Cardiovascular diseases

The solubility of the chromium (VI) compound in water and the particle size are determining factors for the health risks. 

Demonstrating the presence of chromium (VI) in the body

Exposure to chromium (VI) in the distant past cannot be demonstrated via a medical examination. 

Scientific studies have investigated whether chromium (VI) compounds can be detected in the urine, blood, hair, nails, bone, saliva, teeth, respiratory tract and lungs. Chromium (VI) is rapidly converted into chromium (III) – in just a few hours or days – so demonstrating the presence of chromium (VI) quickly becomes impossible. Showing that chromium is present in the blood and urine can therefore only be done shortly after exposure to chromium (VI). 

The analysis of chromium in bodily fluids and tissues generally involves determining the total amount of chromium, which effectively means the concentration of chromium (III) compounds, given that neither metallic chromium, nor compounds containing chromium that is hexavalent, or in other oxidation states, are normally present at significant levels.

Chromium (VI) compounds accumulate in the red and white blood cells where they are converted to chromium (III). The chromium (III) compounds are then no longer able to leave the cell. Consequently, the total chromium content gives an indication for absorption of chromium (VI) compounds that has occurred during the lifespans of red and white blood cells (4 months and 1 month respectively). In work environments, this analytical method is used to monitor workers up to 4 months after their exposure. Determining chromium levels in urine and blood cannot be achieved if a number of years has passed since the time of exposure.

Increased chromium levels can be found in the hair and nails up to one year after exposure. However, it is impossible to establish that there is a link between the chromium content in the hair or nails and the level of exposure or its source. 

Bone is theoretically suited for examination of an elevated exposure to chromium for up to 15 years after exposure. The major drawback is that obtaining bone samples (and other specific tissue samples) is highly invasive; biopsies during surgery or autopsy are basically the only option. Also in this case, it is impossible to establish a link between the measured concentrations in bone and other tissues, and the level, duration and source of exposure because of the daily exposure and conversion to chromium (III). It is therefore not possible to use the measured concentrations as a basis for estimating the risks to health caused by possible occupational exposure to chromium (VI). 

Chromium (VI) does not accumulate in the dental matrix in adults. 

In cases where the exposure consisted of particles containing chromium (VI) that remained unchanged in the lungs, it may be possible to demonstrate the presence of specific chromium (VI) compounds. Studies among workers have found chromium (III) and chromium (VI) in respiratory condensate at the end of the working week. This may, in the future, be a non-invasive alternative to taking lung tissue samples for demonstrating exposure to chromium (VI) in the longer term. However, more research is needed. 

None of the analyses mentioned above allows conclusions to be drawn about how long ago the chromium compounds were absorbed and over what period, nor whether it concerned hexavalent chromium compounds. There are no reference values that the analysis results can be checked against, nor can any prognostic statements be made about the health risks caused by chromium (VI) exposure.