evaluating the male reproductive system. Traditionally measured characteristics of semen samples include sperm concentration, motility, and structure. Oligospermia is defined as sperm concentration below the reference value of 20 million sperm/mL, asthenospermia as less than 50% motile sperm with forward progression, and teratospermia as less than 30% sperm with normal head structure (Rowe et al., 1993). A number of factors may adversely influence male fertility, including sexually transmitted and other diseases (such as diabetes, tuberculosis, and mumps), prolonged high fever, some drugs and medications (such as hormone treatments and cimetidine), some injuries, and some occupational exposures (such as to lead and to the pesticide dibromochloropropane) (Rowe et al., 1993). Another means of indirectly evaluating the impact of potentially toxic exposures on reproductive health is to assess hormone levels in either male or female subjects.

Infertility is the failure to conceive after at least 12 months of unprotected intercourse (Rowe et al., 1993). It has been estimated that 10–15% of couples of reproductive age experience some form of infertility (Speroff et al., 1999). In the general population, the probability that a couple engaging in unprotected intercourse will conceive in the first month is 30%; about half of all couples will conceive within 2 months, and 80% in 6 months (Joffe, 1997). There are numerous risk factors for infertility, including advanced age and obesity in women; previous reproductive experiences; genetic factors; diseases such as chlamydial infection in women or epididymitis in men; and, to a lesser extent, cigarette smoking, alcohol consumption, and toxic agents in environmental and occupational settings in either sex (Templeton, 2000).

A frequently used measure of infertility is time-to-pregnancy (TTP). TTP studies examine the number of months or menstrual cycles that are required to conceive. The results of TTP studies are often expressed as fecundability ratios. Fecundability refers to the probability of conceiving within one menstrual cycle and is a population-based measure that is useful in the quantitative analysis of fertility potential (Speroff et al., 1999). A fecundability ratio (FR) is the ratio of the probability of conception in an exposed group with that in a comparison group. Decreases in the fecundability ratio indicate longer time to pregnancy for the exposed group. Some studies use a conditional fecundability ratio (CFR), which includes only couples that have conceived a child.

Epidemiologic Studies of Preconception End Points and Exposure to Insecticides

Sperm and Semen Characteristics

Only a few studies have examined the relationship between insecticide use and semen characteristics. Two cross-sectional studies were conducted on a small cohort of men who were employed for 1–8 years in a carbaryl production and packaging plant. In the first study, by Whorton and colleagues (1979), 47 current and past carbaryl workers with at least 1 year of work in carbaryl production and packaging were compared with a control group of 90 male chemical-plant workers. The carbaryl-plant workers were divided into three exposure groups (high, medium, and low exposure) on the basis of frequency of exposure and job classification. Each participant was interviewed, provided a semen sample, and underwent a physical examination. The study found a greater proportion of oligospermic men among the carbaryl workers than among the chemical workers (14.9% and 5.5%, respectively, p=0.07). In further analyses by job classification and exposure group, the study found that 16% of the 25 men in the high-exposure group were oligospermic, compared with 13.6% of the 22 men in the low- and medium-exposure

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