Is Bad Air Making You Infertile?

https://www.sciencedirect.com/science/article/pii/S0147651325013004

We frequently hear about the environmental impacts of air pollution—how it contributes to climate change or irritates our lungs. However, a growing body of research is pointing toward another, much more intimate consequence of the air we breathe: its effect on human reproduction.

A 2025 study published in the journal Ecotoxicology and Environmental Safety has taken a closer look at how ambient air pollution impacts male fertility. By examining the exact timeline of how sperm develops, researchers have uncovered fascinating—and concerning—details about how low-to-moderate air pollution can alter sperm quality.

Infertility is much more common than many people realize. Infertility is common, affecting approximately 15 % of couples attempting pregnancy, and men are estimated to contribute up to 50 % of cases. Evidence suggests there has been a 50 % decrease in sperm count over the past four decades.

This dramatic decline has left scientists searching for answers. While lifestyle and genetics play major roles, researchers are increasingly concerned about the growing burden of environmental factors.

Specifically, they are looking at ambient air pollution, like fine particulate matter (PM2.5) and nitrogen dioxide (NO2), which have been associated with lower sperm concentration, motility, and normal shape.

To understand the connection between the air we breathe and male fertility, researchers turned to a unique location: the Salt Lake City metropolitan area in Utah.

Salt Lake City frequently exceeds guidelines set by the Environmental Protection Agency (EPA) for safe air quality. During the winter, geographic temperature inversions trap fine particulate matter (PM2.5) close to the ground, and during the summer, the region experiences high levels of ozone (O3). This makes it a highly vulnerable region and a prime location to study the health impacts of air pollution.

The researchers looked at 1,965 men who were partners in couples seeking infertility treatment between 2013 and 2018. Using advanced geographic models from the National Air Quality Forecast Capability, they estimated the daily levels of specific pollutants at each participant's home address. They then matched this pollution data with the results of four separate semen samples provided by each man over a six-month period.

Timing is Everything

To truly understand the study's findings, we have to look at how sperm is created. The process is called spermatogenesis, and it is essentially a highly sensitive, 74-day biological assembly line.

Because sperm cells are rapidly dividing and transforming, they are highly vulnerable to environmental stressors, like oxidative stress caused by air pollution. The researchers broke this 74-day window into four distinct developmental stages to see when pollution does the most damage.

If a man breathes in high levels of pollution during Day 40, it might affect the genetic division of his sperm. If he breathes it in on Day 5, it might affect how well those sperm can swim.

Which Pollutants Do the Most Damage?

The researchers tracked four specific air pollutants: PM2.5 (fine dust and soot), Nitrogen Dioxide (NO2), Sulfur Dioxide (SO2), and Ozone (O3). They discovered that different pollutants affect the sperm assembly line at different times.

The Threat of Ozone (O3)

Ozone in the upper atmosphere protects us from the sun, but ground-level ozone—created when pollutants from cars and factories react with sunlight—is highly toxic.

The study found that exposure to ozone across the entire 74-day window was associated with a higher percentage of abnormally shaped sperm.

An increase of just 5 parts per billion (ppb) of ozone during the Meiosis stage was linked to a 6.73% drop in normally shaped sperm. The same increase during the Spermiogenesis phase was linked to a 3.83% drop in normally shaped sperm. Ozone exposure during the final Spermiation phase was associated with a 3.66% decrease in overall sperm count.

The Threat of Fine Particulate Matter (PM2.5)

PM2.5 consists of tiny, inhalable particles that are 2.5 micrometers or smaller. These particles can easily enter the lungs and the bloodstream.

The study found that PM2.5 does its worst damage at the very end of the sperm development cycle. Higher exposure to PM2.5 during the final Spermiation window was associated with a lower sperm concentration. It was also linked to lower progressive motility—meaning the sperm are less capable of swimming forward properly.

Exposure to air pollution early in the sperm development cycle generally results in poorly shaped sperm, while exposure late in the cycle tends to result in lower sperm counts and poorer swimming ability.

The "Pollutant Mix" and Seasonal Differences

One of the most complex parts of studying environmental health is that we don't breathe in just one chemical at a time; we breathe in a soup of different pollutants.

Initially, when looking at individual pollutants in isolation, the numbers suggested that PM2.5 might actually improve sperm shape. However, the researchers realized this was a statistical illusion. When they adjusted their models to account for the entire mixture of pollutants together, they found that ozone (O3) was the true driving force behind the damage.

This became especially obvious when researchers looked at the seasons.

Warm Seasons: Ozone levels are significantly higher in the summer. During warmer months, higher ozone was strongly associated with poorer sperm shape, while higher PM2.5 was associated with poorer swimming ability (motility).

Cold Seasons: Despite the notorious winter temperature inversions in Salt Lake City that trap PM2.5, the researchers found fewer clear, negative associations between PM2.5 and semen quality during the colder months.

We frequently assume that air quality is only dangerous when the sky is visibly thick with smog. However, this study proves that chronic exposure to low-to-moderate levels of ambient air pollution can have tangible, negative impacts on human reproduction.

For couples struggling with infertility, the focus is often placed on diet, exercise, and medical interventions. While these are critical, this study highlights that our external environment plays a massive, undeniable role in our internal biology. Advocating for stricter air quality regulations is not just an environmental issue; it is a fundamental healthcare and reproductive rights issue.