A Japanese government study of 94 men shows that only 4% of men in their 20s have sperm counts that meet World Health Organisation fertility standards.

This compares with 9% of men aged 37 to 53.

The study measured the structure, density, liveliness and survival rate of sperm.

The sperm met the WHO minimum standard of 20 million sperm per millimetre of semen. But the activity of the sperm - important in conception - was well below the WHO's 50% level.

Men in their 20s had a 27.2% average, compared with a 28% average for older males.

Chemical pollution

It is not known what causes the fall in sperm counts and whether fertility is affected, but some experts say chemical pollution could be a factor.

Sperm levels are also dropping in other highly industrialised countries in Europe and North America.

Are Japanese men facing a sperm crisis?

Japan's Health and Welfare Ministry says there is still not enough research data to show any link between pollution and fertility.

But the Japanese Environmental Agency has this week increased environmental standards on chemicals which pollute the water supply. It has also added another 300 chemicals to its list of substances suspected of affecting human and other lifeforms and will be monitoring them closely.

Environmental concerns have recently come to the fore in Japan, after decades of rapid economic development.