INTRODUCTION
Childhood cancer has become a significant public health issue, with an overall increase in incidence rates by 1% every year in the past three decades 1. Life style changes and exposure to various environmental factors are thought to contribute this trend besides better cancer diagnosis. One of these environmental factors is cigarette smoke, which contains approximately 7000 chemical compounds, 72 of them are carcinogenic 2. Main source of cigarette smoke exposure of children is parental smoking.Various chemicals related to cigarette use have been detected in fetal blood, placenta, human breast milk and tissue samples of infants including urine, blood and hair3. However, numerous studies have encountered challenges in establishing a clear correlation between tobacco smoke exposure and the incidence of childhood cancers. The application of questionnaires, which are the most used method may potentially introduce bias in studies. Cotinine is the main metabolite of nicotine which is the most abundant organic compound of cigarette and is a reliable biomarker of tobacco smoke exposure. Hair cotinine has longer half-time than the other body samples and hair cotinine analysis is an efficient way to detect chronic exposure. In this study, we used both questionnaire and hair cotinine analysis to detect the extent of environmental tobacco smoke (ETS) exposure among cancer patients and establish the relationship between ETS and childhood cancer.