The study released on Wednesday by the Health and Welfare Ministry examined the density, activity, form and survival rate of the sperm of 94 Japanese men and compared them to World Health Organisation (WHO) standards.

Only 4 per cent of the men in their 20s had sperm that met WHO minimums. The young men were bested only slightly by subjects aged 37-53, who met the standards 9 per cent of the time.

Other countries are facing similar problems. Recent studies in the United States, Europe and elsewhere have shown a fall in the sperm count.

Even so, the Ministry says research still is too limited to draw any wide conclusions. Scientists cannot yet determine whether the declining sperm quality would have an impact on fertility, or whether it is directly related to pollution. "The sperm activity rate was very low for all age groups ...But we don't know the cause of that yet," wrote the study's author, Dr Shigeru Oshio of Teikyo university medical department in Tokyo.

The sperm activity rate -- a key to conception -- was far below the WHO's 50 per cent. The subjects in their 20s had an average rate of 27.2 per cent.