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TABLE 8Case-Control Studies of Paternal EffectsType of Exposure/OccupationAssociation withExposure a Effect Ref.Printing industry (+) Cleft lip 144Paint (+) Cleft palate 144Paint + Damage to CNS b 145Solvents (+) Damage to CNS b 145Low-level radiation + Neural tube defects 146Organic solvents + Spontaneous abortion 147Aromatic hydrocarbons + Spontaneous abortion 147Dusts + Spontaneous abortion 147Radiation + Childhood leukemia 148Welding + Time to conception 149Agriculture (+) Child brain tumor 150Construction (+) Child brain tumor 150Food/tobacco processing (+) Child brain tumor 150Metal + Child brain tumor 150Lead (+) Spontaneous abortion 151Lead (+) Congenital defects 152Ethylene glycol ether + Abnormal spermiogram 153Metals + Cadmium in semen 154a(+): marginally significant association; +: significant association.bCNS = central nervous system.Source: Modified from Taskinen, H., Scand. J. Work Environ. Health, 16, 297, 1990.and other data sources. Like population-based studies, case-control studies provide thestatistics needed to detect a rare event; however, in this type of study individual bias mayyield misleading results. A person who has experienced an adverse outcome may report anexposure more readily than one with no history of reproductive dysfunction. As Levindescribes, 81 a couple that has recently experienced a stillbirth or congenitally malformed childwill be more inclined to search for a previous toxic exposure as the source. Another potentialsource of bias in these studies exists with the interviewer, who should be “blind” to thereproductive status and the exposure classification of the respondent.2.3.4 Standardized Fertility RatioThe standardized fertility or birth ratio (SFR) (Table 9) provides a comparison of thenumber of observed births within a population to the number of expected births. The lattervalue is obtained using the birth rate of an external population. To monitor a specific workforcegroup, Wong 82 uses a simple standardization method that provides noninvasive, readilyavailable data. His method uses a couple’s reproductive history to measure male fecundity.The number of expected births is obtained from the birth rate for U.S. women of the sameage group. There are some problems with this simplified approach. Because the U.S. birthrate as collected by the National Center for Health Statistics does not take into accountmarital status, birth control, or frequency of intercourse, 83 the potential exists for a positiveor negative bias in making such comparisons. Wong’s method erroneously assumes all womento be equally libidinous and fecund. Additionally, because only married men are evaluated inSFR studies, there are age-related restrictions in the data as well as a likely underestimate ofthe number of children born out of wedlock. Some researchers have tried to correct for thesevariations in the population using different models. 83,84 It appears that the SFR overestimatesbirth rate in both control and exposed populations, even when a reduction in semen quality© 1999 by CRC Press LLC
TABLE 9Standarized Birth Ratio Studies of <strong>Male</strong> Fertility EffectsType of Exposure/OccupationAssociation withExposure aRef.Ethylene dibromide (+) 82DBCP + 83TDA and DNT – 78Mercury – 155Manganese + 155Ethylene glycol ether – 85Chemical-contaminated sewage – 156a–: no significant association; (+): marginally significantassociation; +: significant association.Source: Modified from Taskinen, H., Scand. J. Work Environ.Health, 16, 297, 1990.exists. 85 Levine 84 suggests SFR would be a reliable indicator in a cohort study in which bothcontrol and comparison population are selected on the same basis.2.3.5 Cohort StudiesCohort studies (Table 10) evaluate the frequency of adverse reproductive outcomebetween exposed and unexposed groups. Cross-sectional cohort studies evaluate the groupsas they currently exist, whereas historical cohort studies are conducted on previously exposedand unexposed groups. An example of the latter is the 1989 Vietnam experience study inwhich military veterans were grouped according to whether or not they had served in Vietnamfrom 1967 to 1972. 86 This study was able to detect subsequent differences between thegroups in semen quality and time to pregnancy but revealed little about the reproductivehealth of the individuals at the time of exposure.A cohort study may be longitudinal (prospective). In such a study, baseline data arecollected, and individuals are studied over time for a specific reproductive outcome. Alongitudinal study is exemplified by the study of “summer hire” pesticide applicators. 41Individuals were evaluated at the beginning of the season before they started working withpesticides and at the conclusion of the spraying season 2 months later. This report illustratesthe importance of selecting appropriate variables for a study design. If semen analyses areconducted to predict reproductive outcome, the correct time frame is needed. Since the timefor spermatogenesis and delivery of mature sperm to the ejaculate is approximately 72 days,if primary spermatogonia were affected by exposure, this would not be observed in a timeframe covering less than 80 to 90 days. Thus, the study of “summer hire” workers could notmake valid conclusions regarding the effects of a 2-month exposure to pesticide applicationon spermatogenesis.Cohort studies may involve questionnaires, neuroendocrine measurements, and semenanalyses or a combination of these. Questionnaire studies are advantageous in that they arethe least expensive and least invasive. As with case control studies, however, inherent problemsexist, such as possible interviewer bias and the selective memory of respondents who haveexperienced an adverse outcome. Additionally, many cohort studies lack the number ofpregnancies needed to detect differences between groups. 87 Studies evaluating the neuroendocrinesystem measure male hormones in blood samples. Although blood sampling is awidely accepted medical practice, it is an invasive procedure, and the endocrine profile doesnot necessarily reflect the status of the male reproductive system. 88 Semen analysis provides© 1999 by CRC Press LLC
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