Authors’ response

Sir,

I thank Drs Dhara and Acquilla for their interest in my article1 and appreciate their suggestion to analyze the exposure-response relationship using the distance of individual residence at the time of disaster. The surrogate markers used in earlier studies to determine the severity of MIC exposure are: death of family members or neighbours, immediate symptoms and severity of symptoms, hospitalization, perceptions of the presence of MIC and locality of residence at the time of disaster2. Except for the distance of residence from the Union Carbide Plant, the collection of other parameters from illiterate (61% population in severely affected area 41% in moderate to mildly affected area were illiterate)2 after two decade are subjected to high recall bias. The effect of distance of residence was further influenced by additional variables, e.g. duration of exposure, protective measures used by them at that time (i.e. cover of face, splashing of water on face, etc.), and their activity after the disaster (i.e. ran away from their houses, stayed inside their house). Therefore, the distance of individual residence at the time of disaster alone may be inadequate to assess the severity to MIC exposure. The approximate distance of residence of our study population was calculated using their registration numbers (registration numbers were assigned as per satellite heath centers nearer to their original residence) and no differences in prevalence of different lung function abnormalities was observed.

The IMCB survey3 reported high respiratory morbidity among exposed survivors and documented its relationship with the distance of residence from the Plant. The FEF_25--75% was significantly reduced (84% of predicted) among those who were residing nearer to the plant. However, no such association was observed for the other important indices of lung function i.e. FEV₁, FVC and FEV₁/FVC. The FEF_25-75%, is a variable spirometric parameter to assess small airway function and it depends upon the subjects’ expiratory effort during spirometry4. Therefore, the FEF_25-75% values were not utilized in this study1.

Unlike previous studies, the result demonstrated the high prevalence of obstructive lung function abnormalities among gas victims1. The observation was based on retrospective analysis of spirometry data and hence the direct exposure-response relationship could not be evaluated. It was postulated that cumulative effect of other known confounding factors for developing obstructive lung function i.e. smoking, low economic conditions might have influenced the observations. Even after excluding the current and ex smoker from both the groups, the relative risks in our study remained similar. The community based study using these results will be required to determine the true association of MIC and lung function abnormalities.