An environmental pollutant can change the ratio of sperm carrying male and female chromosomes, a study says.
A baby's sex is determined by X and Y chromosomes in the sperm.
Researchers found exposure to a class of pollutants which are a by-product of industrial and agricultural processes increased the Y chromosome sperms.
But the team from Lund University in Sweden, which studied 149 fishermen, were unable to predict if the effect would lead to more boys being born.
Lead researcher Professor Aleksander Giwercman said a larger population sample would be needed to confirm that.
But he added: "We think the fact that exposure to environmentally derived chemicals can change the sex chromosome ratio in sperm is worrying in itself and requires more attention from scientists and the public."
The researchers analysed the effect of exposure to two persistent organochlorine pollutants - DDE and CB-153 - which is most likely to come from eating fatty fish such as salmon.
The 20% of men with the highest exposure to DDE compared with the 20% with the lowest exposure had 1.6% more sperm with the Y chromosome, the Human Reproduction journal reported.
For CB-153 there was a 0.8% increase.
Dr Allan Pacey, a specialist in male fertility at the University of Sheffield and secretary of the British Fertility Society, said: "It is the first time I have heard of pollutants having this effect.
"If this is so it is possible others could have a similar impact and that could have huge consequences."
A second study has also suggested environmental factors can have an effect on male reproductive health.
The joint Denmark, Finland and Lithuanian study, also published in Human Reproduction, showed an incident rate of undescended testes, which increases the risk of testicular cancer, in Lithuania of 5.7%.
This was lower than the 9% rate in Denmark, but higher than the 2.4% rate in Finland.
Data on semen quality and testicular cancer in the Nordic-Baltic region had led the researchers to expect similar rates of undescended testes in Lithuanian and Finnish boys.
The fact that this was not so indicated that something in the environment was having an effect on male foetuses developing in the womb.
Report co-author Dr Niels Jorgensen said: "We need to look more closely at the role of environmental factors, including those that can disrupt the hormone system, and the role of genetics, lifestyle and other factors."
Peter Baker, director of the Men's Heath Forum, said pollutants had been linked to a whole range of problems.
"This is an area that needs much more research."