After determining that the number of confirmed disease cases is in excess of what is expected and reviewing the literature to identify potential causes, the public health department will evaluate how exposures might have occurred. The public health department conducts an exposure assessment, which obtains basic information about the proposed exposure sources. For example, the public health department might investigate the following questions:

• What chemicals, physical hazards, or biologic hazards are present?
• What are the biologic effects of these chemicals or hazards? Have any been associated in reputable scientific literature with the disease(s) of concern?
• Are there known or potential pathways by which these chemicals or hazards might have impacted the population at risk? What were the likely doses that resulted from such exposures?

In many cases, patient concerns focus on an exposure "source"-such as a landfill, dump, or manufacturing facility-rather than on one or more specific exposures that might have occurred.