Chromium can be highly toxic, with the skin and upper respiratory tract particular targets. As Lisa Bushby discusses, the airborne concentration of chromium must be monitored if specified processes with chromium are carried out, and the health of people exposed to chromium or its compounds should be monitored by a health professional to look for early signs of sensitisation.
Chromium is a metal that occurs naturally in the environment in the form of metal-ores such as those containing iron. Naturally-occurring chromium exists predominantly in its chromic (III) form, while the chromate (VI) state is produced mainly from commercial and industrial processes. Not all forms of chromium present the same risks, however, with hexavalent chromium compounds being the most hazardous. The toxicity of chromium (VI) compounds (chromates, dichromates and chromic acid) is highly dependent on their solubility, with the least soluble compounds, such as lead chromates or barium chromate, being the least toxic of these.
Chromium is used in a wide variety of industrial processes such as in the tanning of leather, in the manufacture of special alloys, as wood preservative, pigments in paints, plastics, ceramics and glass and as decorative plating for taps and door handles, as a high temperature catalyst, and as a dietary supplement.
Soluble hexavalent chromium compounds, in particular chromium trioxide, are used extensively as electrolytes in electroplating and anodising. In these processes, the bursting of small bubbles of gases generated by electrolysis causes the formation of mists composed of fine droplets of electrolyte, known as chromium plating mists.
Even single exposures to hexavalent chromium compounds can cause irritation and inflammation of the nose and upper respiratory tract if such compounds are in the air, and irritation of the skin with skin contact. Exposure to chromic acid can cause burns to the skin, possibly leading to ulcers and eye damage. Repeated exposure to hexavalent chromium compounds, meanwhile, can cause damage to the nose, including ulcers and holes in the flap of tissue separating the nostrils (the nasal septum), inflammation of the lungs, allergic reactions in the skin and respiratory tract, kidney damage and cancer of the lung. In addition, based on experimental data, there are concerns about the potential effects on reproduction, relating both to male fertility and the development of unborn babies.
Requirement for monitoring
Owing to the significant health risks, Regulation 10(2) of the Control of Substances Hazardous to Health Regulations 2002 (COSHH) requires that monitoring is carried out to demonstrate that control measures are properly used and maintained for specific substances and processes listed in schedule 4 of the regulations. “Spray given off from vessels at which an electrolytic chromium process is carried out, except trivalent chromium” is one of the processes listed with a requirement that monitoring is carried out every 14 days.
The method described in the HSE’s MDHS52/3: Hexavalent Chromium in Chromium Plating Mists, as follows, is recommended to measure the concentration of hexavalent chromium in the air above or adjacent to chromium plating baths, and to demonstrate that this is below the workplace exposure limit of 0.05 mg/m3 in any case.
Sampling times in the range 15 minutes to 2 hours are recommended for measurements made using a spectrophotometer or colorimeter, but a minimum sampling time of one hour should be used when measurements are made using a comparator.
A sample of chromium plating mist should be collected by drawing a measured volume of air through a sodium hydroxide-treated membrane filter mounted in an inhalable sampler. The filter should then be leached with dilute sulphuric acid to dissolve any soluble hexavalent chromium compounds present, and the colorimetric reagent (1,5-diphenylcarbazide solution) mixed with the sample solution. After a 10-minute delay to allow the colour to develop, the sample should be measured with a spectrophotometer, colorimeter or comparator. If a spectrophotometer or colorimeter is used to measure the colour developed (at 540 nm), hexavalent chromium in air concentrations are determined by a calibration curve technique. If a comparator is used, hexavalent chromium in air concentrations are estimated by visual comparison of the colour developed with standard colour discs. The comparator method, therefore, has the advantage of simplicity, but yields less precise results.
The method can also be used for measuring personal exposure to hexavalent chromium. However, personal exposures are normally substantially lower than the lower limit of the working range of the method unless control of chromium plating mist emissions is poor. It is therefore recommended that, where possible, personal exposure to hexavalent chromium is assessed by measuring total chromium. Provided that control measures are properly applied, used and maintained, they should be capable of keeping the concentration of hexavalent chromium in the air above or adjacent to chromium plating baths well below this level.
Hexavalent chromium compounds may be absorbed into the body following inhalation exposure, although the extent of absorption depends on solubility. Some absorption may also occur following ingestion and there is also some absorption of hexavalent chromium across the skin. Once absorbed, hexavalent chromium binds to red blood cells, and may remain in the body for several weeks, before elimination as chromium (III), mainly in the urine.
The health of people exposed to chromium or its compounds should be monitored by an occupational health professional who will carry out skin inspections of hands and forearms to look for chrome ulceration and rashes that may indicate chromium sensitisation and who will looks for signs of nasal irritation/perforation. They will normally carry out an examination after an offer of employment is made and at regular intervals during employment thereafter. As chromium is excreted in urine, a urine sample is sometimes required so its chromium content can be measured.
In addition, employees should examine their own skin regularly, looking for colour changes, rashes and skin damage, which should be referred to a health professional.
Poisoning by chromium is a prescribed disease in the UK, which means there may be an entitlement under the Industrial Industries Disablement Benefit.
COSHH control measures
Under COSHH, with respect to chromium, employers are required to:
assess risks to health and the precautions needed to protect from those risks
prevent exposure to chromium, or where this cannot reasonably be done, adequately control exposure
reduce exposure to airborne chromium and its compounds so far as reasonably practicable, and in any case to below the workplace exposure limits (WELs) assigned for chromium and its compounds:
for chromium (VI) compounds, 0.05mg/m3 of air averaged over an 8-hour period
for other chromium compounds, 0.5mg/m3 of air averaged over an 8-hour period.
maintain all fume and dust controls in efficient working order
provide fit testing of any tight-fitting respirators
determine how much chromium workers are exposed to, normally through a monitoring programme, and inform them of the results
arrange any health checks that are necessary
inform, instruct and train all employees who may be exposed to chromium about the risks and how they are being controlled.
Chromium: health effects, incident management and toxicology, Public Health England, available from gov.uk.
EH2: Health Surveillance for Hexavalent Chromium Compounds, HSE, available from the HSE website.
INDG436: Chromium and you, HSE, available from the HSE website.
MDHS52/3: Hexavalent chromium in chromium plating mists, HSE, available from the HSE website.