Background: Exposure to microorganisms could be caused infectious diseases but some exposures were not caused disease in normal situation. Objective of this study was to determine the health effects of biological hazards in low to moderate concentration. Methods: It was a study. The people who were employed in different industries were participated in this study. Groups were followed for infections in skin, respiratory, mucous membranes, gastrointestinal, urinary and etc. symptoms and signs. These groups were exposed to biological hazards; according to working sections, 3 groups were participated: working section A, working section B and working section C. Symptoms and signs were determined. Data were analyzed with SPSS 16. ANOVA, Chi-2, Exact test and relative risks with considering P<0.05 as significant level. Results: Working section B, had the most microorganisms; in air: 5.01±0.01 CFU on surface: 10.15±0.01 CFU. Infection in skin, respiratory, mucous membranes, gastrointestinal, urinary and etc symptoms and sign were determined. Skin, gastrointestinal, urinary and etc were the most in group B and relative risks were shown significant risks. Relative risks for skin infection were 1.80(1.12-3.15). Conclusions: Even in normal situation microorganism in workplace could be caused many infection diseases and prevention was necessary.