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Chemicals which affect the blood can do so either by being directly toxic to blood cells, or by preventing it from delivering oxygen to the rest of the body. Haemoglobin is the molecule which carries oxygen to the body's tissues, and by combining with it to form carboxyhaemoglobin, carbon monoxide prevents oxygen reaching the tissues. Carbon monoxide is produced when organic substances such as gas and petrol are incompletely burnt, and as it is present in car exhausts, fire fighters, garage workers and traffic police can be exposed to high concentrations. Carbon monoxide is also produced by poorly maintained gas fires, and can also be produced in the body by metabolism of some chemicals like methylene chloride. Blood carboxyhaemoglobin levels are around 0.5% in non-smokers, and 5% in smokers. Headaches occur at levels of 10-20%, and a level of 60-70% is fatal within hours.

Other substances combine with haemoglobin to form methaemoglobin, which is also unable to deliver oxygen to the tissues. Workplace chemicals which have this effect include aniline dyes, nitrous gases, potassium chlorate, nitrobenzenes, phenylenediamine, and toluenediamine. Methaemoglobinaemia also results from environmental exposures to foods high in nitrates or nitrites, or well water contaminated with nitrates (Levy and Wegman).

As well as binding with red blood cells to interfere with the carriage of oxygen, other chemicals can break up red blood cells. This is known as haemolysis, and is caused by chemicals like naphthalene, copper, organic compounds of metals such as tributyltin, and arsine.

Arsine gas is an inorganic arsenic compound, and although it is used in the electronics industry, it is most dangerous because it can be formed accidentally when arsenic-contaminated metals or coal come into contact with acids. There have been cases of sewer workers being gassed by arsine after using acids to clear drains previously contaminated with substances containing arsenic. As well as its effects on red blood cells, arsine also causes kidney failure.

Lead also affects the blood by interfering with red blood cell manufacture. These cells are produced in lower quantities and do not live as long as normal, causing anaemia. Workers exposed to lead, such as welders, painters, jewellers, and in smelters, foundries and potteries, are covered by special legislation in the UK and should have biological monitoring to assess their exposure.

Benzene is another well known example of a chemical which causes blood toxicity. It is metabolised in the body to a chemical which damages bone marrow, the site where blood cells are produced. Leukaemia, a form of cancer, was first linked with exposure to benzene in the 1920s. It also causes aplastic anaemia which, like leukaemia, is often fatal.

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