Of the overall 400 participants, 225 (57%) were women and 175 (43%) were men. There were statistically significant differences in the distribution of age, years of education, location of house, self-reported health impact and the number of affected household members between the three groups. The mean age of subjects in control group A (46 years) was higher than the other two groups (exposed group, 35 years; control group B, 31 years) due to a higher proportion of subjects in the > 50 age group and a smaller proportion in the 15–30 age group. Control group B subjects had higher mean years of schooling (12.7 years) than the exposed group (10.3 years) and control group A (11.0 years). A higher proportion of the exposed group (53%) lived in houses on the main road compared to control group A (22%) or B (18%). A higher proportion of the exposed group (78%), a lower proportion of control group A (53%) and a still lower proportion of control group B (24%) reported that they were worried about the possible effects of the oil spill on their health. A higher proportion of exposed group (81%) believed that oil spill had affected their health, while a lower proportion of unexposed group A (23%) and still lower proportion of unexposed group B (3%) believed the same. Differences in smoking habits, passive smoking status, past history of allergy to chemicals, dust and pollen, and past history of wheezing and asthma between the groups were not significantly different (table ).
Selected characteristics, medical history and beliefs of study participants according to exposure category for health effect of oil spill in Karachi, Pakistan 2003
Ninety-six percent of the exposed group (207/216), 70% (58/83) of control group A and 85% (86/101) of control group B reported one or more symptoms (P < 0.001). Mean symptoms score based on the presence of 48 symptoms in a particular subject were higher among the exposed group (14.1) compared to control A (4.4) or control B (3.8). Further, mean score for each sub-group of symptoms were higher among exposed as compared to control groups. Mean score from nine ocular symptoms among exposed group were 3.4 as compared to1.1 and 0.7 for control groups A and B respectively. Mean score from seven upper respiratory tract symptoms among exposed were 3.1 as compared to 1.1 among each of control group A and B. Mean score from the five skin related symptoms were 1.0 for exposed and 0.1 and 0.1 for each of control group A and B. Mean score from 17 nervous system related symptoms was 4.3 for exposed as compared to 1.2 and 1.3 for control group A and B respectively.
We compared the proportion of each self-reported symptom among the exposed and the control groups, and found that they were highest for the exposed group located closest to the spill, followed by control group A and then B. Proportion of those who reported ever having wheezing was not different among the exposed 8.3% (18/204), the control group A 10.3% (9/84) and the control group B 5% (5/101; P = 0.329). Further, exposed 13/204 (6.0 %) who ever had episodes of wheezing that made them short of breath before the spill was not significantly different from control group A 7.2% (6/84) and control group B 3.0% (3/101, P = 0.400). After the spill, 6% (13/204) had wheezing with shortness of breath among the exposed while only 1.2% (1/82) among the control A and no one in control B.
Only 32.9 % (68/207) of symptomatic subjects in the exposed group and 29.6% (16/54) of control group A, and 19.0% (16/84) of subjects in control group B consulted a health care provider for the symptoms. Sixty-six percent (45/68) of those who consulted a health care provider in the exposed group reported only one visit. Majority (67%) of the symptomatic subjects in the exposed group reported that they had recovered completely or were recovering, whereas the remaining 33% reported that their illness was persistent.
Logistic regression model adjusted for the effect of age, gender, education, smoking, dust allergy, chemical allergy, house on the main road, and anxiousness about health effect of oil spill indicated that there was statistically significant moderate -to-strong associations (PORs ranging from 2.3 to 37.0) between the exposed group and the symptoms listed above and also reported in table . There was a trend of decreasing symptom-specific PORs with decrease in exposure (Table ).
Crude and adjusted a prevalence odds ratio (POR) and corresponding 95% confidence interval (CI) for specific symptoms by exposure category group
To assess those who were more affected among the exposed, we constructed logistic regression models for each symptom as dependent variable and included age, sex, building of residence, floor of apartment, house on the main road, and presence of windows towards sea as independent variables. We found that for itchy eyes, scratchy throat, sore throat, nausea/vomiting, headache females were more likely to experience symptoms as compared to males.
Multiple linear regression model revealed strong relationship of exposure with the symptoms score (β = 8.24, 95% CI: 6.37 – 10.12, Table ).
Multiple linear regression model of relationship between exposure status and the symptoms score
A higher proportion of those who had symptoms among the exposed group 54.1% (112/207) reported interference of symptoms with their daily routines as compared to 26.8% (15/56) in control group A or 3.5% (3/86) in control group B. The average number of days that the symptomatic subjects were not able to work was significantly higher among exposed group (2.9 ± 6.3) than control A (1.0 ± 3.4) or control B (0.2 ± 1.5, P < 0.001).