Dangerous substances are found in many workplaces. A recent survey found that 16 % of workers in Europe reported handling hazardous products and 22 % being exposed to toxic vapours. Exposure to dangerous substances can occur anywhere at work, on farms, in hairdressers’ shops, in motor-vehicle repair shops, at chemical plants. Dangerous substances can cause many different types of harm. Some cause cancer, others can affect the ability to reproduce or cause birth defects. Other substances may cause brain damage, harm to the nervous system, asthma, and skin problems. The harm done by dangerous substances can occur from a single short exposure or by the long-term accumulation of substances in the body. The theme for the European Week for Safety and Health at Work 2003 is the prevention of risks from dangerous substances. The Agency is producing a series of factsheets focusing on the communication of occupational safety and health-related information on dangerous substances including biological agents. This factsheet introduces the key issues in this topic.