Sreeramareddy et al. BMC Public Health 2011, 11:403 http://www.biomedcentral.com/1471-2458/11/403 LBW. Two systematic reviews have reported about the possible association between air pollution and LBW [11,12]. Providing evidence for the association between these risk factors and LBW may form the basis for plan- ning intervention strategies. Recently, there has been a growing interest about health effects of exposure to biomass smoke [8,13]. Indoor air, which is polluted from burning biomass fuels, may contain carbon monoxide (CO), carbon dioxide (CO2), nitrogen dioxide (NO2), sulphur dioxide (SO2), and volatile organic compounds (VOCs) and particulate material [13-15]. Studies have linked exposure to indoor/ biomass smoke with respiratory infections, tuberculosis, cataract, cardiovascular events and also LBW [16]. Smoke emitted from burning biomass fuels contains a large number of air pollutants that have adverse health outcomes [17,18]. In developing countries, a large num- ber of households still depend on biomass fuels for cook- ing and space heating [13,17]. Biomass fuels are usually burnt on inefficient, unvented household cooking stoves often in poorly ventilated houses or kitchens [9]. Studies from Pakistan [19], Zimbabwe [20], Guatemala [21] and India [22] have reported about possible association of maternal exposure to biomass smoke with reduced birth weight. A study from Guatemala reported that babies born to women who cooked with wood were on an aver- age 63 grams lighter than babies born to mothers who were using either gas or electricity for cooking [21]. Another report based on Zimbabwe Demographic and Health Survey (1999) found that the babies born to mothers who cooked using wood, dung or straw were on average 175 grams lighter than babies born to mothers who cooked with LPG, natural gas or electricity [20]. A retrospective cohort study from Pakistan reported that babies born to mothers who cooked with wood were on average 82 grams lighter than the babies born to mothers who used natural gas [19]. A prospective cohort study from rural south India has reported that exposure to bio- mass fuel was associated with 49% increased risk of LBW [22]. Though the epidemiological studies mentioned above varied in their designs, they have shown consistent results. A systematic review about the effect of indoor air pollution on LBW has also concluded that the asso- ciation is consistent despite the limited evidence from epidemiological studies [23]. We aimed to further test the hypothesis that exposure to smoke from burning biomass fuels during pregnancy lowers the birth weight using a large nationally representative sample of house- holds from 2005/6 Demographic Health Survey (DHS) of India. Our objective was to test the association between use of biomass fuels and mother’s self-reported size of the child at birth. Page 2 of 10 Methods We conducted a secondary data analysis of the most recent DHS of India. India DHS 2005-06 [also called National Family Health Survey, (NFHS-3)] was carried out under scientific and administrative supervision of International Institute for Population Sciences (IPPS), Mumbai and Macro International during the time period November 2005 to August 2006. The survey collected information about demographic factors, socio-economic factors and health status from a nationally representative probability sample of households. The sample size was 109,041 households selected by two-stage probability proportional to size (PPS) method in rural areas and three stage PPS sampling in urban areas. In both urban and rural areas, primary sampling units were blocks/ wards or villages. From each of the chosen sampling units (i.e. village/s or block) random household sampling was done. Within each selected household, all women aged 15 to 49 years were eligible to be respondents for the survey. As primary sampling unit was household, a national household weighing factor was used to maximise representativeness of the sample. Data were collected according to a standard protocol. Three core survey questionnaires i.e. the Household Ques- tionnaire, the Woman’s Questionnaire and the Man’s Questionnaire were translated into 18 local languages and field tested. Subsequently, filled questionnaires were back- translated to English. These questionnaires were used in all 29 states of India. Questionnaires used in each state were bilingual, with questions in both principal languages of the state and English. To minimize language barriers, the survey was administered by trained interviewers either in English or principal language of the state or preferred language of the household. Further details of sampling design, training of survey team, survey management and quality control measures are separately documented in the country reports published by ORC (Opinion Research Corp.) Macro International [24]. From a sample of 109,041 households a total of 124, 385 female participants were interviewed giving a response rate of 94.5%. For our analyses, the respondents were ever married women aged between 15 to 49 years who had given birth to at least one child. All these participants were asked about births during five years prior to the survey (interview) date. For each birth, details about place of birth, birth weight, gender and size of the baby at birth were asked. Ethical considerations and consent The Independent Review Boards (IRB) of IPPS and ORC Macro international had independently reviewed the DHS protocols, data collection tools and procedures and provided ethical approval. Approval was also taken from the IRBs of Melaka-Manipal Medical College, Malaysia