Your gym shoes may be harboring bacteria, fungi, and pathogens that affect your health. Here is what research says is actually growing inside athletic footwear.

Are Gym Shoes Making You Sick? What's Actually Growing Inside Them

Most people wash their gym clothes after every workout. Almost nobody treats their gym shoes with the same regularity.

That asymmetry matters more than most people realise. The inside of a well-used athletic shoe is one of the more biologically active surfaces you encounter in daily life — and it's in sustained, direct contact with your skin for hours at a time.

What the Research Says

The most cited study on athletic footwear contamination comes from the University of Arizona, where microbiologist Dr. Charles Gerba and his team tested bacterial content across a range of everyday surfaces in 2008. Shoe soles averaged 421,000 bacteria per shoe — compared to approximately 1,000 on a typical toilet seat. The contrast became a frequently cited data point precisely because it disrupted the intuitive sense of what's "clean" and what isn't.

The inside of the shoe was not the focus of that study, but subsequent research into the shoe interior tells a more directly health-relevant story. The insole and lining of an athletic shoe create near-ideal conditions for microbial growth: temperature of 85–95°F during wear, humidity approaching 100% during exercise, a constant supply of dead skin cells and sweat as nutrients, and no light exposure. Bacterial populations under these conditions can double every 20–30 minutes in the hours immediately post-workout.

A single workout can deposit roughly 1 gram of skin cells and several hundred millilitres of sweat into each shoe. Bacteria metabolise this material continuously between uses.

What's Actually Growing In There

The microbial community inside an athletic shoe is more complex than most people expect. It includes bacteria, fungi, and in certain contexts, more concerning organisms.

Bacteria are the most abundant residents. *Staphylococcus epidermidis* and *S. aureus* are common — the latter being the species responsible for a range of skin infections including folliculitis, impetigo, and in vulnerable individuals, more serious complications. *Corynebacterium* species are responsible for a significant portion of foot odor. *Pseudomonas aeruginosa*, found in some studies of athletic footwear, is notable for its antibiotic resistance profile and its ability to cause infection through breaks in skin.

Fungi — particularly dermatophytes like *Trichophyton rubrum* — colonise the insole and lining and are the direct cause of athlete's foot (tinea pedis) and fungal nail infections (onychomycosis). Once established in a shoe, they survive for weeks to months in foam and fabric, re-exposing the foot with every wear.

Viruses receive less research attention in this context, but the human papillomavirus (HPV) strains responsible for plantar warts survive on surfaces and can transfer from contaminated footwear to skin through minor abrasions. Shared gym environments create transmission opportunities.

When It Becomes a Health Problem

For healthy adults with intact skin, the microbial load in an untreated shoe typically produces foot odor and increases the probability of fungal infections — persistent but manageable conditions for most people.

The picture changes in specific circumstances:

*Recurring fungal infections* are directly linked to shoe contamination. Studies on tinea pedis recurrence consistently identify contaminated footwear as a primary source of re-infection following treatment. Treating the skin without treating the shoe is the most common reason athlete's foot returns.

*Skin breaks and abrasions* change the risk profile significantly. Blisters, cuts, or cracked skin provide entry points for bacteria that would otherwise be blocked by intact skin. Athletes who train intensively often have minor skin compromise from friction or callus management — conditions that make the bacterial environment inside a shoe more directly relevant.

*Immunocompromised individuals* — those on immunosuppressive medications, people with diabetes-related immune changes, or anyone with compromised circulation in the lower extremities — face meaningfully higher risk from the organisms found in contaminated footwear.

*Shared footwear* — including rental shoes at climbing gyms, bowling alleys, and skate rinks — creates a direct transmission pathway. When hundreds of different people wear the same shoes without effective sanitisation between uses, the microbial accumulation accelerates.

The Comparison That Puts It in Perspective

Gym shoes are often worn for hours, stored in enclosed bags with residual sweat, and worn again without any cleaning between sessions — a pattern that would seem extraordinary applied to any other piece of clothing. The reason it's normalised for shoes is partly practical (shoes can't go in the washing machine the way a t-shirt can) and partly because the inside of a shoe is invisible.

Research from the University of Houston found bacterial contamination levels on gym surfaces including equipment handles and locker room floors. Athletic footwear picks up organisms from these surfaces and carries them into personal spaces. The floor contamination most people are conscious of is, in a meaningful sense, delivered into the shoe interior and worn against the skin.

What To Do About It

The solution is straightforward, if not always convenient with traditional methods.

Air dry after every session. Remove insoles and open the tongue of the shoe to maximise airflow. The reduction in residual moisture slows bacterial growth between uses.

Rotate between at least two pairs of athletic footwear. This allows each pair to dry fully before the next use and reduces the intensity of contamination buildup in any single pair.

Replace footwear regularly. Athletic shoes worn for training accumulate contamination over months. By 400–600 miles of use, foam insoles are compressed and structurally degraded in ways that make deep cleaning less effective. Replacing heavily worn shoes is sometimes the most practical hygiene intervention.

For active fungal infections or recurring athlete's foot, sanitise after every wear. This is the one behaviour change most consistently associated with breaking the recurrence cycle in clinical recommendations.

Medical-grade sanitisation — UV-C light combined with ozone — reaches bacterial and fungal contamination throughout the shoe structure, including the foam layers that air drying and surface sprays cannot address. At fitness venues with Freshtrax kiosks, this is available in 90 seconds as part of a normal training session.

The Bottom Line

Your gym shoes are not going to send you to the hospital. But they are significantly more contaminated than most people assume, and for anyone dealing with recurring foot infections, skin issues, or training in shared footwear environments, the contamination level has real health consequences.

The fix is the same principle as washing your clothes: regular, consistent hygiene rather than occasional cleaning after visible problems develop.

*Freshtrax brings medical-grade shoe sanitisation to fitness venues → [See where Freshtrax is available](/how-it-works)*