Competing interests
The authors declare that they no competing interests.
Authors’ contributions
GHS was the main investigator, collected the data, performed the statistical analysis, and drafted the manuscript. SN and AHM supervised the study and participated in the design of the study, final revised of manuscript and intellectual helping for analyzing of data. KY, RN, and MA were advisors of the study and carried out statistical and technical analysis of data, participated in design of study and manuscript preparation. All authors read and approved the final manuscript.

This research was part of a PhD dissertation of the first author and has been supported by Tehran University of Medical Sciences under grant no. 92-03-46-24084. The authors express their gratitude to all laboratory staff of the Department of Environmental Health Engineering.