This study aimed to test the hypothesis that exposure to cooking with biomass fuels, such as wood or dry leaves, cow dung and rice straw, is significantly associated with the development of cataract among adults less than 50 years of age in rural Bangladesh. After adjusting for family history of cataract, smoking status and level of formal education, differences in exposure to the various biomass fuels were found. Cooking with rice straw was identified as being positively associated with young adult cataract, whereas cooking with cow dung was negatively associated.
The positive association of cooking with rice straw with case status was stronger in comparison with NE than with NC controls. A possible, though unsupported, explanation for this difference depending on the type of controls employed is that the other eye diseases share these risk factors with cataract patients, or that patients with diseases included among the non-eye-disease control group are less exposed to these particular cooking fuels. Inter-comparison of the two types of control, however, revealed no significant associations with the use of these cooking fuels.
It is important to note that, in our study, the association of young adult cataract with using rice straw for cooking among females is relative to the use of other biomass fuels. Because of the sex differences in the patterns of exposure to cooking and the fuels used, the comparator group for males was no exposure to cooking. In contrast, other studies have examined the associations between cataract and the use of cheaper, biomass or solid fuels relative to the use of clean fuels. The use of less expensive cooking fuels was more common among patients with age-related cataract than non-cataract patients in India  and, compared with the use of stoves burning clean fuels such as biogas, liquefied petroleum gas or kerosene, the use of solid fuel in unvented stoves was associated with cataract among females of any age in the Nepal-India border area . In both studies the associations remained significant after adjustment for other risk factors, including low educational achievement.
A plausible mechanism underlying the association between the use of certain biomass fuels and development of cataract may be related to the constituents of the large amounts of smoke produced from these fuels damaging the tissues of the eye following either systemic absorption or even local diffusion through the cornea. It has been suggested that such damage may be a result of the endogenous generation of reactive oxygen species by photodynamic action, similar to the purported mechanism by which smoking tobacco may raise the risk of cataract [12, 13].
Despite the relationship between risk of cataract and use of cooking fuel being reported in a number of studies, few have attempted to document the associations for different types of biomass fuel. The component materials have either not been specified or have been specified but combined in the analysis and comparisons made with the use of clean fuels. Thus, Mohan (1989) and Mishra (1999) reported elevated risks of both cataract and blindness among an Indian population with exposure to the smoke of biomass cooking fuel, specified as wood, crop residuals and/or cow dung, but separate analyses of each of these materials were not described [14, 15].
In view of these reports, the independent inverse association between the use of cow dung as a cooking fuel and case status in our study was unexpected. The relationship held true in comparisons with each type of control. While copious amounts of smoke are known to emanate from burning cow dung, and indoor burning of dung has been reported to produce higher PM10 concentrations than that of wood or straw , the opposite directions of relationship in our study of rice straw and cow dung might be related to the different complement of smoke constituents. Although cow diet consists largely of fresh grass, bacterial and enzymatic actions of the bovine gastrointestinal tract result in considerable transformations of the plant material.
Constituents of smoke from biomass fuels have been reported to vary considerably with the type of stove employed and with various other differences in the way the fuel is prepared. Comparative information on the constituents of smoke from different biomass fuels, or from dung fuel separate from other biofuels, is scarcely available in the scientific literature, despite several studies of smoke constituents of biomass fuels combined [17–21]. Mudway (2005), however, demonstrated that particles derived from the burning of cow dung cake burned in a traditional Indian cooking stove and deposited in the human respiratory tract lining fluid had considerable oxidative activity, which was mostly due to their transitional metal content . If the postulated mechanism whereby smoke from biomass fuels induces cataract formation through the activity of reactive oxygen species is true, then it is difficult to understand why smoke from cow dung does not have a positive association with young cataract, similar to that of rice straw. Further comparative analyses are required to identify differences in the smoke constituents and elucidate possible differences in the mechanisms of action.
Consideration, however, must also be given to the possibility that the apparent protective effect against the development of young adult cataract of using cow dung as a cooking fuel could be due to uncontrolled confounding. Exposure to cow dung as cooking fuel is more common among middle class families in rural areas in Bangladesh. Cows are usually used for cultivation and dairy products, so more frequently kept by land and farm owners, whereas poor families can rarely afford to buy or keep cattle. Use of cow dung as a fuel thus may be acting as a proxy for higher socioeconomic status, which itself has been identified in some previous studies to be associated with a lower prevalence of cataract (of any type) [1, 6, 23–25]. Nevertheless, adjusting our models for family income level or for the composite socioeconomic status indicator based on the Kuppuswami scale had no discernable effect on the relationship between case status and use of cow dung as a cooking fuel, so that confounding, if it is to be invoked as the explanation for the relationship, must involve an as yet unidentified variable.
It is of interest, however, that an Indian study of the relationship between fuel use and ocular morbidity in which separate independent associations between different types of cooking fuel and cataract were examined reported a significantly increased risk for wood but not for cattle dung or for gas, kerosene or coal . On the other hand, eye irritation was significantly associated with the use of coal and cattle dung but not the other fuels.
Other variables related to case status in our study - family history of cataract, a history of cigarette smoking, and low educational attainment - have each been recognized as risk factors for cataract in other studies [27–30]. The relationship with low educational achievement may be explained by the generally poorer nutritional status of less educated people. Poor nutritional status  as well as experience of dehydrational crises , have been identified as independent risk factors for cataract. Unlike the findings of some previous studies , working in sunlight was not identified as being associated with case status.
A limitation of this study stems from difficulties in recalling lifetime use of various cooking fuels, although recall was stimulated during the interview by referring to significant life events of each patient. However, it is unlikely that recall misinformation was differential as all patients, both cases and controls, were visiting the hospital for treatment of some ailment, and the specific hypothesis under study was not known to the subjects. Such random errors that may have occurred would therefore tend to reduce the observed strength of association between exposures and outcome. Interviewer knowledge of subject status and the hypothesis under study, which could theoretically introduce bias and an overestimation of associations, is unlikely to have introduced significant distortion of the data as the interviews were carried out strictly according to the structured questionnaire.
Variables of exposure to cooking fuels in this study were confined to those subjects who did the cooking. As other family members could also be exposed, albeit probably to a lower extent, the associations might have been underestimated. Indeed it has been shown that the PM10 concentrations in living rooms were only slightly lower than, and closely followed, those in the kitchen throughout the day, in poor households in Bangladesh . Unfortunately, in our study, it is not known whether those subjects who were not exposed to cooking generally remained in the house while cooking was done or were at work away from home.
Both the case-control design of the study and the fact that the relationship between use of rice straw as cooking fuels and development of young adult cataract was not consistently significant in comparisons with the two controls, preclude our drawing firm conclusions regarding a causal relationship between the cooking with rice straw and the risk of developing young adult cataract. The inverse relationship between the use of cow dung and case status, however, was much more consistent. Nevertheless, a plausible explanation for the association is lacking.
The study did not classify the cases with respect to type of cataract. Unless all types share the same risk factors, any heterogeneity of cataract types would have the effect of diluting the true relationships with exposure.
Finally, since this study was conducted in a charitable non-government organization hospital, catering for disadvantaged villagers in remote parts of the country, the range of socio-economic status among subjects was not very wide. Such restricted variability may have prevented the identification of certain risk factors that may be seen in studies with a wider variety of patient backgrounds. On the other hand, the location of the hospital at the time of the study in a poorly accessible rural district with only meagre permanent healthcare facilities makes it unlikely that the cases and controls were drawn from different catchment populations.
The strength of this study lies in its separation of different types of traditional cooking fuel, which allowed the identification of contrasting directions of association among these fuel types.