By Amy Norton

NEW YORK (Reuters Health) - Mother Nature has always ensured that male births outnumber female ones, but the gap has been gradually narrowing over the past three decades in the U.S. and Japan, according to a new study.

Researchers suspect the decline in male births can be explained, at least in part, by paternal exposure to environmental toxins, such as certain pesticides, heavy metals, solvents or dioxins -- chemical byproducts produced during incineration or the manufacture of other chemicals.

Traditionally, it's been expected that for every 100 girls born, there will be about 105 boys. This balances out the higher death rate among male fetuses and infants. But since 1970, the U.S. and Japan have experienced a downward shift in this male-to-female birth ratio, researchers report in the online edition of the journal Environmental Health Perspectives.

In the U.S., the proportion of boys dropped from 105.5 per 100 girls in 1970 to 104.6 in 2001; in Japan, the male-to-female ratio dropped from 106.3 boys for every 100 girls to just fewer than 105 per 100.

The changes may seem small, but the study authors suspect they are one manifestation of the effects of environmental pollutants on the male reproductive system.

The decline in male births has occurred "at the same time that we've been seeing other signs that male reproductive health is in danger," said lead study author Dr. Devra Lee Davis, a professor of epidemiology at the University of Pittsburgh Graduate School of Public Health.

These other signs, she told Reuters Health, include lower testosterone levels and sperm counts, as well as increases in testicular cancer, a disease that most often affects young men.

Environmental toxins may be a common denominator here, according to Davis and her colleagues. Such exposures may specifically lower rates of male, rather than female, births for a few reasons. They may, for example, affect the viability of sperm that bear the Y chromosome, which determines male sex -- or the viability of male fetuses.

Davis's team found that while fetal deaths have declined overall in recent decades, the proportion of male deaths is growing. In Japan, in particular, male fetuses accounted for about two thirds of all fetal deaths in the 1990s.

Over the years, there have been a number of reports showing that heavy exposure to certain pollutants may affect a man's likelihood of fathering a son.

Men in the Italian town of Seveso who were exposed to large amounts of dioxin through an industrial explosion in 1976 fathered significantly more girls than boys. Similarly, a study of workers at a Russian herbicide plant found that only 38 percent of children born to male workers were boys; female workers, on the other hand, had the expected ratio of male-to-female children.

It's not known whether chronic low-level exposure to toxic chemicals could have similar reproductive effects, according to Davis. But it's important to find out what's behind the decline in male births, she and her colleagues point out.

"The question is, what...level of evidence do we need before we take action," Davis said.

For now, she recommends that people who want to limit their everyday exposure to potentially harmful chemicals read the labels of the household products they buy. For example, she said, "avoid things that say 'fragrance' but don't tell you what it is."

Alternatives include using the various "green" products on the market, as well as old-fashioned cleaning standbys like baking soda and vinegar.

SOURCE: Environmental Health Perspectives, online April 9, 2007.