Around one in six people suffer from infertility worldwide.
And since more than half of the world’s population currently lives in urban areas, researchers are interested in understanding whether living in noisy, polluted cities might be to blame.
A in Denmark used nationwide data to explore infertility.
It found that long-term exposure to air pollution and traffic noise may be associated with increased infertility, but these factors affect men and women differently.
What do pollution and noise do to the body?
We know that traffic pollution has an undeniable impact on the environment. Its negative effects on human health are also well established, with links to cancer and heart disease.
Chemicals inhaled from polluted air can also reach the reproductive tract through the blood. They can reduce fertility by disrupting hormones or causing direct damage to eggs and sperm.
The effects of traffic noise on health are less clear, but some research suggests that it affects stress hormones, which could alter fertility.
What did the study look at?
This new study was conducted in Denmark, which collects data on each resident in multiple national databases over their lifetime, using a unique identification number.
National-level data allows researchers to investigate links between a person’s health and factors such as where they live, work, education and family. This method is called “data linking”.
The study aimed to capture people who were likely trying to get pregnant and, therefore, at risk of receiving an infertility diagnosis.
Over two million men and women have been identified as being of reproductive age. The study looked at those who were:
- from 30 to 45 years
- cohabiting or married
- with fewer than two children
- live in Denmark between 1 January 2000 and 31 December 2017.
All those diagnosed with infertility before the age of 30, living alone or in a registered same-sex partnership are excluded. People with incomplete information (such as a missing address) were also excluded.
There were 377,850 women and 526,056 men who met these criteria.
The study did not examine them. Instead, over a five-year period, it cross-checked detailed information about where they lived and whether they had received an infertility diagnosis collected from the Danish national patient registry.
The researchers also estimated how much each residential address was exposed to road traffic noise (measured in decibels) and air pollution, or how much fine particulate matter (called PM2.5) was in the air.
What did the researchers discover?
Infertility was diagnosed in 16,172 men (out of 526,056) and 22,672 women (out of 377,850).
The study found that the risk of infertility was 24% higher for men exposed to PM2.5 levels 1.6 times higher than those recommended by the World Health Organization.
For women, exposure to traffic noise 10.2 decibels above average (55-60 decibels) was associated with a 14% increased risk of infertility for people over 35.
The risks were similar whether you lived in urban or rural areas and whether you took education and income into account.
What does it suggest?
The study highlights how environmental exposure can have immediate and long-term effects and can affect male and female reproduction differently.
After puberty, men constantly produce sperm: up to 300 million per day. The impact of environmental changes on male fertility – such as exposure to toxic pollutants – tends to occur more rapidly than in females, affecting the number and quality of sperm.
In contrast, women are born with all their eggs and cannot produce new ones. Eggs have some “damage control” mechanisms to protect them from environmental risks throughout life.
This does not mean that eggs are not susceptible to damage. It may take longer than the five years of exposure examined by this study for the impact on women to become clear.
It is possible that long-term studies may also reveal a similar impact of pollution on women.
Is data linking a good way to look at fertility?
Data linkage can be a powerful tool for uncovering links between environmental exposures and health. This allows evaluations on large numbers of people, over long periods of time, such as this recent Danish study.
But there are inherent limitations to this type of study. Without examining individuals or considering biological factors – such as hormone levels and body mass – the research is based on a few assumptions.
For example, this study made some important assumptions about whether or not couples were actually trying to conceive.
It also calculated people’s exposure to noise and air pollution based on their address, assuming they were at home.
A more precise picture could be drawn if information were collected from individuals on their exposure and experiences, including regarding fertility.
For example, surveys could include factors such as sleep disturbances and stress, which can alter hormonal responses and impact fertility. Exposure to hormone-disrupting chemicals is also found at home, in everyday household and personal care products.
In its scope, this study is unprecedented and constitutes a useful step in exploring the potential link between air pollution, traffic noise and infertility. However, more controlled studies – involving actual exposure measures rather than estimates – would be needed to deepen our understanding of how these factors affect men and women.
Amy L Winship is a team leader and senior research fellow in Anatomy and Developmental Biology at Monash University. Amy Winship receives funding from the Rebecca Cooper Medical Research Foundation, Cancer Council Victoria and Monash University.
Mark Green is an Associate Professor and Merck Serono Senior Lecturer in Reproductive Biology at the University of Melbourne. He is also deputy scientific director of research at Monash IVF.
