While infertility may be caused by a number of factors, new study findings suggest that exposure to nonpersistent, or short-lasting, insecticides may play a role in male infertility.

“Environmental exposure to chlorpyrifos or its metabolite (TCPY) may be associated with reduced levels of circulating testosterone in adult men,” lead study author Dr. John D. Meeker, of the University of Michigan, Ann Arbor, told Reuters Health. “A decline in testosterone throughout a population could potentially lead to adverse reproductive health outcomes,” he added.

Until 2000, chlorpyrifos was one of the most common insecticides used in homes. The Environmental Protection Agency restricted its residential use to reduce children’s exposure to the chemical after research revealed it can affect the central nervous system.

Just one year earlier, however, up to 19 million pounds of the chlorpyrifos were used in the United States, and recent investigations suggest that individuals still experience environmental exposure to the substance, despite EPA restrictions.

The Second National Report on Human Exposure to Environmental Chemicals found that more than 90 percent of men had detectable levels TCPY in their urine.

In a previous report, Meeker and his colleagues found that higher levels of 1-naphthol (1N) in men’s urine are associated with decreased sperm concentration and motility and increased DNA damage in sperm cells. 1N is a breakdown product of carbaryl, a lawn and garden insecticide known as Sevin, and the compound naphthalene, which is found in cigarette smoke, diesel fuel and other combustion byproducts.

Meeker and his colleagues from the Harvard School of Public Health in Boston and the Centers for Disease Control and Prevention in Atlanta explored the association between TCPY and 1N, and reproductive hormone levels in 268 men who were recruited at an infertility clinic between 2000 and 2003.

Men with higher urine levels of TCPY and 1N had lower levels of the sex hormone testosterone, the researchers report in the journal Epidemiology. The association with TCPY was dose-dependent, such that testosterone levels decreased along with increasing levels of TCPY.

Higher TCPY levels were also associated with a decreased free androgen index, a marker of lower testosterone concentrations, the report indicates.

“Although the decrements in testosterone related to TCPY were relatively small,” Meeker acknowledged, “they may be of public health concern because of widespread human exposure among men.”

The researchers also found some evidence that higher TCPY and 1N levels may be associated with decreased levels of luteinizing hormone and a decreased free androgen index, but more studies are needed to confirm this finding.

If TCPY and 1N are associated with decreased levels of both testosterone and luteinizing hormone, the hypothalamus and the pituitary gland, rather than the testes, may be involved in the mechanism by which certain pesticides affect sperm quality, Meeker speculates. “But there are several other potential mechanisms as well,” he said.

According to Meeker, “This is the first human evidence of an association between chlorpyrifos or its metabolite (TCPY) and testosterone levels, so other studies would be needed to substantiate our findings.”

SOURCE: Epidemiology, January 2006.