The composition of municipal solid waste is quite heterogeneous. This mixed composition results in the presence of a variety of microorganisms that reach densities which are relatively high, and which remain high even after many years in a landfill. Microorganism densities in the air at municipal solid waste processing plants tend to be higher than levels near wastewater treatment facilities. This may be due to indoor operations. Composting can inactivate essentially all of the microorganisms associated with fecal matter, but thermophilic fungi may cause adverse health problems. The presence of microorganisms in municipal solid waste does not mean that there is a high risk of infection or disease if a person is occupationally exposed to the waste. Dose-response relationships with various microorganisms have shown that a relatively large number of microbes are necessary to initiate an infection or cause disease.