Because prolonged exposure to harmful substances can cause chronic ill health, and symptoms do not always develop immediately.
For example, long-term use of two-pack spray paints containing a fixer called isocyanates can cause chronic asthma. The condition can develop suddenly, when you become 'sensitised' to the chemicals. The fine airborne mist involved in unprotected, uncontrolled spraying with two-pack paints is one of the main causes of occupational asthma in the UK.
