A European study shows that environmental pollutants may affect sperm.
The study shows that men with high exposure to certain chemicals have sperm that could alter the ratio of the sexes.
The study was relatively small and didn't flag any particular chemical to watch out for. The researchers say more work is needed and it's not clear when any environmental influences may take effect.
The study comes from Aleksander Giwercman of Sweden's Lund University and appears in the online edition of Human Reproduction.
Pollutants May Influence Sperm
The Swedish study centered on persistent organochlorine pollutants (POPs). POPs are found throughout the environment, say the researchers.
POPs are related to domestic and industrial discharge, automobile exhaust, street run-off, slum sewage, and agricultural chemicals, says the journal's news release.
POPs build up in the environment. In Sweden, people are typically exposed to POPs by eating contaminated fatty fish, says the study. Swedish fishermen eat on average twice as much fatty fish as the general population.
The study included 149 fishermen. Sperm and blood samples from the fishermen showed that the men with more exposure to POPs had a slightly higher proportion of sperm bearing "Y" chromosomes.
Girls are born by fertilization of an egg with an "X" chromosome-carrying sperm. Boys are born through fertilization of an egg with a Y-carrying sperm.
The researchers say their results indicate that exposure to POPs may influence sperms' Y:X ratio but the results do not provide circumstantial evidence about the proportion of male births.
Recent reports in many countries indicate that the proportion of male births has been declining and may be in part due to exposures to POPs, say the researchers.
One might think that that would mean a lower proportion of Y-carrying sperm of men exposed to POPs. Fewer boys, fewer Y chromosomes, in other words.
However, that's the opposite of what was seen in the Swedish fishermen.
Changing the Boy-Girl Ratio?
The researchers say the higher Y:X ratio of chromosome-carrying sperm in the fishermen's ejaculate didn't necessarily translate into how many boys or girls the fishermen fathered. Many factors can affect which sperm fertilizes an egg and ultimately affect the sex ratio of a population, including the timing of intercourse.
It's also not clear if the Y:X ratios of the ejaculate stay stable over time, write researchers.
SOURCES: Tiido, T. Human Reproduction, April 28, 2005; online edition.