Occupational hygiene is discipline recognizing, evaluating and controlling occupational health hazards and about practical risk assessment in the workplace (exposure assessment). The risk assessment in principle consists essentially of an identification of the present hazards and an estimate of the extent of the risks involved taking into account whatever precautions already have been taken. Many think of the term „risk assessment“ as belonging primarily to the environmental community; however, if you are a practicing industrial hygiene professional, you are already doing risk assessment. Risk assessment has been broadly defined as the methodology that predicts the likelihood of numerous unwanted events, including industrial explosions, workplace injurie, natural catastrophes, injury of death due to an array of voluntary activities (e.g., skiing, sky driving), diseases (e.g., cancer, developmental toxicity caused by chemical exposure), death due to natural causes (e.g., heart attack, cancer) and death caused by lifestyle choices (e.g., smoking, alcoholism, diet). As a tool for health risk assessment a system of categorization of work operations has been established in the Czech Republic.