An occupational hazard is a hazard experienced in the workplace. This encompasses many types of hazards, including chemical hazards, biological hazards (biohazards), psychosocial hazards, and physical hazards. In the United States, the National Institute for Occupational Safety and Health (NIOSH) conduct workplace investigations and research addressing workplace health and safety hazards resulting in guidelines. The Occupational Safety and Health Administration (OSHA) establishes enforceable standards to prevent workplace injuries and illnesses. In the EU, a similar role is taken by EU-OSHA.

Occupational hazard, as a term, signifies both long-term and short-term risks associated with the workplace environment. It is a field of study within occupational safety and health and public health. Short term risks may include physical injury (e.g., eye, back, head, etc.), while long-term risks may be an increased risk of developing occupational disease, such as cancer or heart disease. In general, adverse health effects caused by short term risks are reversible, while those caused by long term risks are irreversible.

Chemical hazards are a subtype of occupational hazards that involve a wide variety of chemicals. Exposure to chemicals in the workplace can cause acute or long-term detrimental health effects. There are many classifications of hazardous chemicals, including neurotoxins, immune agents, dermatologic agents, carcinogens, reproductive toxins, systemic toxins, asthmagens, pneumoconiotic agents, and sensitizers.

NIOSH sets recommended exposure limits (REL) as well as recommends preventative measures on specific chemicals in order to reduce or eliminate negative health effects from exposure to those chemicals. Additionally, NIOSH keeps an index of chemical hazards based on their chemical name, Chemical Abstracts Service Registry Number (CAS No.), and RTECS Number. Furthermore, OSHA has set permissible exposure limits (PEL) on around 500 chemicals which are enforceable by law.

These exposure limits include evidence that a certain amount of a chemical exposure is linked to one or more adverse health effects. For instance, heart disease is more prevalent in workers who are exposed to the chemicals found in engine exhausts. Exposure to carbon tetrachloride has shown to cause liver and kidney damage. Exposure to benzene has been linked to leukemia.

Biological agents, which create biological hazards, include bacteria, fungi, viruses, microorganisms, and toxins. These biological agents can cause adverse health effects in workers. Influenza is an example of a biological hazard which affects a broad population of workers.

Exposure to toxins generated by insects, spiders, snakes, scorpions, etc., require physical contact be made between the worker and the living organism. Skin exposure to biological agents can cause contact dermatitis (caused by exposure to urushiol from poisonous Toxicodendron plants), Lyme disease, West Nile virus, and coccidioidomycosis (caused by exposure to fungi). According to NIOSH, outdoor workers at risk for these hazards "include farmers, foresters, landscapers, groundskeepers, gardeners, painters, roofers, pavers, construction workers, laborers, mechanics, and any other workers who spend time outside."

Health care professionals are at risk to exposure to blood-borne illnesses (such as HIV, hepatitis B, and hepatitis C) and particularly to emerging infectious diseases, especially when not enough resources are available to control the spread of the disease. Veterinary health workers, including veterinarians, are at risk for exposure to zoonotic disease. Those who do clinical work in the field or in a laboratory risk exposure to West Nile virus if performing necropsies on birds affected by the virus or are otherwise working with infected tissue.