When I said cotton wasn’t the best fabric for sweaty workouts, my comment section filled with one response: “Better than getting cancer from plastic clothes.”

It’s a frightening claim—and one worth taking seriously. But it mixes together three different questions:

Does wearing synthetic clothing actually cause cancer?

Is fiber composition the only factor that determines whether a garment may contain harmful substances?

Is cotton always the safer option?

So, does choosing polyester leggings over cotton ones really put your health at risk? And does “natural” automatically mean safer? The answer is more complicated than either side makes it sound.

The Real Issue With Cotton in Activewear

Cotton can be a great material for casual wear. Depending on its weight and construction, it can feel soft, comfortable and breathable. But it has a weakness—like all of us.

Moisture retention is where cotton falls short during sweaty workouts. Cotton is highly absorbent and generally dries more slowly than common performance fibers. Once you begin sweating, the fabric absorbs that moisture and can remain wet against your skin. Perhaps even your sweat is a fan of cotton.

As you continue exercising, the shirt may become heavier, less comfortable and more likely to leave the equipment you use damp. But discomfort—and the unfortunate wet gym bench—are not the only concerns.

Remaining in damp clothing, particularly when it is tight or rubbing against the body, can increase the likelihood of chafing, irritation, heat rash, body acne and folliculitis, which is inflammation around the hair follicles. Heat, moisture and friction together create the problem; cotton itself does not directly cause these conditions.

The American Academy of Dermatology explains that clothing which traps heat and sweat against the skin can cause irritation and trigger breakouts, particularly when friction is involved. It recommends wearing moisture-wicking clothing and changing out of sweaty workout clothes after exercising.

Does Wearing Polyester Really Cause Cancer?

Polyester is a synthetic, petroleum-derived fiber, so describing it as a type of plastic is not technically wrong. However, “plastic” covers an enormous range of materials, chemicals and applications. The word alone does not tell us how much exposure occurs or whether wearing the finished garment creates a meaningful health risk.

Concerns surrounding textile chemicals and microplastic pollution deserve serious research. However, current evidence has not established that simply wearing ordinary polyester activewear causes cancer.

That is not the same as claiming that every synthetic garment is completely harmless. The safety of a finished product can also depend on its dyes, chemical treatments, manufacturing controls and the amount and route of human exposure.

The International Agency for Research on Cancer evaluates substances and exposures for their potential to cause cancer. It also warns that when something has not been classified, this should not automatically be interpreted as proof that it is completely safe. In other words, we should avoid making absolute claims in either direction.

The Fiber Is Only Part of the Story

A garment’s fiber composition does not reveal every substance used to manufacture it. Dyes, printing chemicals and performance finishes may be applied to natural and synthetic fabrics alike.

Some azo dyes can break down and release carcinogenic aromatic amines, which is why their use in clothing that comes into prolonged contact with the skin is restricted in the European Union. Other textile substances, including certain dyes and formaldehyde, may trigger allergic contact dermatitis in sensitive individuals.

Some water- and stain-resistant textiles may also use PFAS, often called “forever chemicals.” According to the US Environmental Protection Agency, exposure to certain PFAS may be associated with adverse health effects. However, the risk depends on the specific chemical, dose and route of exposure. The presence of a water-repellent finish does not automatically prove that wearing the garment will cause those health effects—but it is reasonable to ask brands which treatments they use.

The same nuance applies to natural materials. “Organic cotton” primarily describes how the cotton fiber was grown. It does not automatically tell consumers everything about the dyes and finishes applied during later stages of production.

Certifications can offer additional reassurance, although no label can promise absolute safety. OEKO-TEX® STANDARD 100 indicates that the finished textile and its individual components have been tested against limits for harmful substances. Bluesign® focuses more broadly on responsible chemical use and manufacturing processes throughout the textile supply chain.

This is why “natural versus synthetic” is far too simple. The fiber matters, but it is only one part of the finished garment.

What Consumers Should Really Ask

Before making your next purchase, instead of asking only, “Is it cotton or polyester?”, consider asking:

What is the full fiber composition?

Does the product make antimicrobial, anti-odor, stain-resistant or water-repellent claims?

Does the brand explain which treatments it uses?

Does it provide credible textile certifications?

Is the garment appropriate for the activity?

Is it durable enough to be worn repeatedly?

Does the brand provide transparent information about its manufacturing standards?

Conclusion

The choice is not “healthy cotton” versus “cancer-causing plastic.” That makes a complicated subject sound frighteningly simple.

Cotton is not a bad fabric, and synthetic activewear is not automatically dangerous. They behave differently, and the fiber name alone does not tell us everything about the chemicals, finishes, quality or safety of a garment.

As a sportswear designer, my original point was about moisture management: cotton absorbs sweat and stays wet, while many performance fabrics are engineered to move moisture away from the skin and dry more quickly.

Health and environmental concerns deserve their own serious conversations, but those conversations should be based on evidence rather than fear.