Occupational asthma is asthma caused by something breathed in at work.

Isocyanates - spray painters, metal or electrical processors, vehicle manufacture and mechanics, plastics/foam workers

Grain or flour - bakers, other food process workers, farmers, farm workers

Wood dusts - wood workers and construction workers

Glutaraldehyde - nurses, darkroom technicians, medical and pharmacological research

Solder/colophony - welders, solderers, electronic assembly

Laboratory animals - laboratory technicians, scientists and assistants, radiographers

Resins and glues - many occupations

Latex - nurses, hospital auxiliaries, laboratory technicians

But there are many more causes of occupational asthma. Some doctors think that occupational asthma in this country is three times as common as the number of cases actually reported. In other words, many people have occupational asthma, and it is never diagnosed or recorded.

The Benefits Agency lists the substances it recognizes will cause asthma.

This is the list:

• Isocyanates
• Platinum salts
• Acid anhydride and amine hardening agents.
• Fumes arising from the use of rosin as a soldering flux.
• Proteolytic enzymes
• Animals including insects and other arthropods of their larval forms used for the purposes of research, education, in laboratories, pest control, or fruit cultivation.
• Dusts arising from barley, oats, rye, wheat or maize, or meal or flour made from such grain.
• Antibiotics
• Cimetidine
• Wood dusts
• Ispaghula
• Castor bean dust
• Ipecacuanha
• Azodicarbonamide
• Glutaraldehyde
• Persulphate salts or henna arising from their use in the hairdressing trade.
• Crustaceans or fish or products arising from these in the food processing industry.
• Reactive dyes.
• Soya bean
• Tea dust
• Green coffee bean dust
• Fumes from stainless steel welding.
• Any other sensitizing agent inhaled at work.

There are more than 200 known respiratory sensitisers. More are found each year.

One of the most dangerous substances and a major cause of occupational asthma are di-isocyanates. In particular, toluene di-isocyanate (TDI) used in many spray-paints, varnishes, polyurethane processes, foams, and waterproofing materials. Making flexible and rigid polyurethane foam, and using twin-pack paints by spraying or brushing, and making flexible packaging in which isocyanates are used as adhesive or ink, are known to be common causes of occupational asthma.

Colophony fumes from soldering cause a significant number of cases of occupational asthma. This comes from pine resin and contains acids, which are used as fluxes in soldering, particularly in electronics.

Laboratory animal allergy is the most serious health risk for pharmaceutical industry workers, and is the most serious health risk for people working in pharmaceuticals and laboratories.

Dust from grain either being harvested or stored is a common cause of asthma. Spores from fungi are released during harvest and the effect is worst in dry weather. Moulds and mites grow when grain is stored. One study of a group of farmers suggested that one fifth had asthma caused by this.

Occupational asthma amongst bakery workers is common. Proteins in the flour often cause this. This is one of the most common causes of occupational asthma.

Enzymes are used widely in industry. They are used in detergent powders, and in the food production and preparation.

Hard wood dusts are a well-known cause of asthma at work. Western red cedar is the best-known cause, but many other woods, including mahogany, obeche and iroko, cause asthma.

Acid anhydrides are another cause. These are used as curing agents in making epoxy and other resins, which are used in paints, plastics and adhesives.

Platinum refining is another risk area, involving complex platinum salts, also used in jewellery. Ammonium hexachloroplatinate is used in refining platinum.

Health workers using glutaraldehye to clean instruments have developed asthma. Formaldehyde is another cause.

Health care workers are also vulnerable to latex allergy through the use of rubber gloves.

Not everyone who is exposed to something capable of causing asthma will go on to develop asthma. Usually, a minority of the workers at risk develops asthma.

You may not develop asthma for weeks, months, or even years after you were first exposed to the cause. Once you have developed asthma, your airways will react to small doses of the substance that caused it. You will have been sensitized to this substance. Up to one half of people are atopic. This means they react to skin prick tests for the most common substances which cause allergy. Some types, but not all types, of occupational asthma, develop more commonly in atopic people.

Cigarette smoking significantly increases your risk of developing asthma if you are exposed to something at work likely to cause asthma.

The more heavily and frequently you are exposed at work to the substance which eventually causes your asthma, the more likely it is that you will go on to develop work-related asthma.

Occupational asthma can become chronic, so that you will suffer from it even after you have left the job that caused it. Whether this happens seems to depend on what sort of substance caused it in the first place, and how long you continued to be exposed to this substance after your symptoms first began.