Breathing in polluted air could damage the fertility of men by reducing how much sperm they produce.
Tests on mice showed those exposed to toxic air had lower counts and worse quality sperm compared to those who had inhaled clean air since birth.
The study looked at particulate matter (PM), tiny particles that penetrate deep into the lungs and enter the blood stream.
It adds to the growing evidence against air pollution and its effect on our health, including the reproductive and respiratory systems.
The World Health Organization (WHO) estimates 15 per cent of the global population has difficulty with fertility.
And male infertility accounts for about half of those problems, according to global estimates.
Researchers at Sao Paulo University, Brazil, looked at four groups of mice. One was exposed to PM2.5 before and after birth until adulthood.
The second group was exposed only during their time in the womb and the third group was exposed after birth from weaning until adulthood.
The fourth group was exposed only to filtered air.
The team, led by Dr Elain Costa, analysed the testes of the mice and their production of sperm when they had become adults.
The mice that had been exposed to toxic air had deterioration in the seminigerous tubes – where sperm is produced in the testes.
The mice that breathed only PM2.5 after birth seemed to be the most harmed, the study found.
The exposure to PM2.5 led to changes in the levels of genes related to testicular cell function.
Dr Costa said these changes are epigenetic, which means they are not caused by changes in the DNA sequence but when a gene is switched on or off.
She believes it to be the first time research has demonstrated that exposure to air pollution impairs production of sperm through epigenetics.
‘These findings provide more evidence that governments need to implement public policies to control air pollution in big cities,’ Dr Costa said.
‘Infertility rates are increasing around the world, and air pollution may be one of the main factors.’
PM2.5 is a fine particle with diameters that are 2.5 micrometers or smaller, making a human hair, at about 70 micrometers in diameter, 30 times larger, or more.
PM2.5 is known to disrupt the endocrine system, involved in reproduction, sleep and metabolism.
It could also harm the function of the genitals. A study published in February suggested men who live on main roads are more likely to have difficulty getting an erection due to exposure to pollution.
Toxic fumes reduced blood flow to the genitals, tests on rats showed, putting them at risk of developing erectile dysfunction.
Numerous scientific studies have linked particle pollution exposure to a variety of problems, including premature death in people with heart or lung disease, heart problems and respiratory problems.