Worldwide, infertility affects approximately one in six people.

With more than half the world’s population now living in urban areas, researchers wonder whether living in noisy and polluted cities could be to blame.

A new study from Denmark used nationwide data to investigate the problem of infertility.

Long-term exposure to air pollution and traffic noise has been found to be associated with increased infertility – but these factors affect men and women differently.

How do pollution and noise affect the body?

We know that road traffic pollution has an undeniable impact on the environment. Its negative impact on human health is also well documented, with links to cancer and heart disease.

Inhaled chemicals from polluted air can also enter the reproductive system through the blood. They can reduce fertility by disrupting hormones or causing direct damage to eggs and sperm.

The health effects of traffic noise are not entirely clear, but some studies suggest it affects stress hormones, which may affect fertility.

What were they looking at?

The new study was conducted in Denmark, where data on each resident is collected in multiple national databases throughout their life, using a unique identification number.

National data allows researchers to examine the links between a person’s health and factors such as where they live, their job, their educational history, and their family. This method is called “data linkage.”

The study aimed to identify people who are likely to be trying to conceive and are therefore at risk of receiving a diagnosis of infertility.

More than 2 million men and women were identified as being of reproductive age. The study included people who were:

• aged 30 – 45
• living together or married
• with less than two children
• lived in Denmark from 1 January 2000 to 31 December 2017.

We excluded anyone who had been diagnosed with infertility before the age of 30, who lived alone or in a same-sex registered partnership, and those with incomplete information (e.g. no address).

These criteria were met by 377,850 women and 526,056 men.

The study did not survey them. Instead, over a five-year period, detailed information about where they lived and whether they had received a diagnosis of infertility was cross-checked, collected from the Danish National Patient Register.

The researchers also estimated how much each residential address was exposed to road traffic noise (measured in decibels) and air pollution, as well as how much fine particulate matter (known as PM2.5) was in the air.

What did they find?

Infertility was diagnosed in 16,172 men (out of 526,056) and 22,672 women (out of 377,850).

The study found the risk of infertility was 24% higher in men exposed to PM2.5 levels that are 1.6 times higher than those recommended by the World Health Organization.

For women, exposure to traffic noise 10.2 decibels higher than average (55–60 decibels) was associated with a 14% increased risk of infertility in those aged 35 and over.

The risk was similar regardless of whether they lived in urban or rural areas, and after taking into account education and income.

What does that suggest?

The study shows that exposure to environmental factors can have immediate and long-term effects and may affect male and female reproduction differently.

After puberty, men continue to produce sperm—up to 300 million per day. The effects of environmental changes on male fertility—such as exposure to toxic pollutants—usually manifest more quickly than in women, affecting sperm count and quality.

Women, on the other hand, are born with all their eggs and cannot produce new ones. The eggs have certain “damage control” mechanisms that protect them from environmental threats throughout their lives.

That doesn’t mean eggs aren’t vulnerable to damage. But it could take longer than the five years of exposure this study looked at for the effects on women to become clear.

It is possible that even longer studies will reveal similar effects of pollution on women.

Is data fusion a good way to study fertility?

Data linkage can be a powerful tool for uncovering links between environmental exposures and health, enabling the assessment of large numbers of people over a long period of time, as in the recent Danish study.

But there are inherent limitations to this type of research. Without studying individuals or looking at biological factors—such as hormone levels and body weight—the research relies on assumptions.

For example, this study made some important assumptions about whether the couples were actually trying to have a baby.

People’s exposure to noise and air pollution was also calculated based on their address, assuming they were at home.

A more accurate picture could be obtained if information was collected from individuals about their exposures and experiences, including in terms of fertility.

For example, surveys may cover factors such as sleep disorders and stress, which can alter hormonal responses and affect fertility. Exposure to chemicals that disrupt hormones also occurs at home, in everyday household and personal care products.

In its scale, this study is unprecedented and is a useful step in exploring the potential link between air pollution, traffic noise, and infertility. However, more controlled studies—with actual measurements of exposure rather than estimates—would be needed to deepen our understanding of how these factors affect men and women.