noses were not verified independently. A team from the University of Texas School of Public Health (UTSPH), led by a physician-epidemiologist, conducted an independent evaluation of these same data from Brio, Tex. The investigators divided the population into 3 zones of potential exposure based on proximity to the waste site and wind patterns in the area. Zone 1 was defined as adjacent to the waste site, Zone 2 was 1,460-3,000 ft (0.4-0.9 km) and downwind from the site, and Zone 3 was 2,100-4,100 ft (0.6-1.2 km) from the site and away from the prevailing winds.
The UTSPH team evaluated 652 household-response forms for various health effects. Respondents reported 121 pregnancies, of which 25 (20.7%) ended in spontaneous abortion. They also evaluated the rates of reported birth defects and used data obtained from the CDC Congenital Malformations Surveillance Report (CDC, 1988) to estimate expected rates. Among the 96 live births, 18 (19%) were reported to have had congenital abnormalities. The investigators attempted to correct for recall bias by using a conservative risk estimate. They assumed that the number of cases recorded in 1990 for about one-third of the area was the annual incidence for the entire population that lived in the area over the period 1983-1989, while also assuming that the medical end points were valid as reported. With this assumption, they found a lower-bound relative risk of 2.4 for congenital birth defects and 3.8 for major CNS malformations. In 181 women in the study 19-50 years old, there were 126 pregnancies, for a fertility rate of 0.7 births per woman per year. This seems very high, even in the absence of a control group, and may indicate a serious problem in the data.
There may have been biases in the ascertainment of cases and confirmation of reported congenital malformations. First, prior to the health survey, extensive media coverage about the site included anecdotal reports about adverse health effects. This may have biased interview responses. Secondly, the survey was conducted by volunteers, and the response rate was low, leading to the possible biases. While there was some attempt to standardize the questioning procedure, there was no recording of home visits and outcomes or of attendance at training sessions. Volunteers may have been especially diligent in seeking positive responses, which would enhance the potential for recall bias. Third, the form was to be filled out by the interviewee rather than the interviewer, which could cause differences in interpretation of the questions and hence increase uncertainty in the answers. Lastly, no attempt was made to confirm the diagnoses by contacting physicians. Thus, there is no independent verification of reported cases. Other factors also inhibited the development of an independent assessment of this problem, including the protracted nature of the dispute, the inability to gather independent information, and the difficulty of obtaining validated measures of exposure.