Occupational exposure of greenhouse workers hasn't yet been studied in Egypt, so that the present work is an endeavor to explore the different potential occupational hazards and their impact on greenhouse workers. Chlormequat, a plant growth regulator, was the main chemical factor affecting workers in nine research greenhouses in Antoniadis Research Branch in Alexandria. Concentrations of chlormequat before, during and after its application were determined. Temperature, percentage relative humidity and heat stress were determined in the nine research greenhouses. Twenty workers and twenty subjects matched control group were examined biologically and clinically. The results obtained revealed that, the treatment increased significantly the total carbohydrate, K and Ca% of leaves content, while the increase of % total chlorophyll was insignificant. Chlormequat concentration during application exceeded much the short term exposure limit [STEL] [728.8 and 300 µg/m3 respectively]. The addition of citric acid as a sticky substance decreased significantly the concentration of chlormequat [P<0.0001]. The residual concentration after five applications slightly exceeded the STEL. Application of chlormequat increased significantly % relative humidity and decreased mean temperature by 2°C. The micronucleus frequency has shown significant increase in exposed workers than in control subjects [P< 0.0001]. The exposed workers might have mutagen induced abnormal DNA repair response. Health problems among exposed workers were mainly asthma, allergic rhinitis, conjunctivitis and dermatitis. The present work highlights the potential occupational hazards in greenhouses and their health effects.