Funky ugly toenails seem to plague runners. Toenail fungus is one cause of thick yellow toenails and it is very common. In fact 14% of Americans have toenail fungus. Based on this, if you are out for a run with six other runners, one of you probably has toenail fungus. 

Although running shoes can put you at risk of toenail fungus there are some simple steps you can take to prevent a toenail fungus infection from your running shoes.

Guns Don’t Kill People...

But flying bullets sure do! Spores are the seeds that give rise to a foot fungus infection. The spores are everywhere. And if they get into your shoes they can grow and multiply. The more fungal spores in your shoes, the more risk of toenail fungus infection. 

Active fungus thrives in any environment that is dark, warm and moist...like the inside of your running shoe. Running shoes are the perfect incubators for toenail fungus! If you get spores in there the active fungus starts to grow and sheds even more spores inside your running shoe. 

The Real Problem...

The worst thing you can do is rush into the house after you finish your run, take off your running shoes and throw them into a dark, poorly ventilated closet. Or if you have been running on the treadmill at the gym you take your shoes and stuff them into your gym bag. 

Your damp running shoes will then sit in the dark and any fungus will thrive in that moisture from your sweaty feet. If you run every day, the running shoes never really completely dry out. These are ripe conditions for toenail fungus. 

The next day you put on your running shoes and head out the door. Not only is there active fungus within your running shoes but you have fungal spores which are basically the seeds for toenail fungus all lining the interior of your running shoes.

So as you are running your toes hit the inside of the running shoes the fungal spores get wedged in between the toenail and the skin underneath. If you have any trauma to your toenails such as banging the end of the running shoe when you're running up or down hill (or stopping quickly) the fungal spores can actually get into the nail bed and start to develop a toenail fungal infection.

Once that happens the toenail fungus actually grows within the keratin of the nail plate separating all of the layers of the keratin and making the toenail thicker.

All of that separation of layers in keratin of the toenail plate causes light to refract unevenly through the toenail. This is what causes the discoloration and crumbling of the toenail that creates the ugly appearance.

While it is possible to treat the toenail infection and kill the fungus, it is critical that you reduce your exposure to the fungus spores if you want to keep your toenails clear.  Remember, you have to stay out of the line of fire if you don’t want to get hit by the bullets. 

You must prevent the accumulation of active toenail fungus and fungal spores in your running shoes. If you don't, it will just be a matter of time before you get a fungal foot infection.

The best defense for a runner against toenail fungus is to make sure that you dry your running shoes out completely. Leave them somewhere where they will dry near a heating vent or in the sunlight.

If you are a high mileage runner and run on back-to-back days it might be a good idea to buy 2 pairs of running shoes. Alternate the running shoes so that they have a chance to dry out completely. This won't be any more expensive because the running shoes will last twice as long.

It is also important to make sure that your running shoes fit correctly. If your toes are bumping the end of the shoes (as evidenced by discoloration such as bruising or dark spots under the toenail) you might actually be traumatizing the toenails when you run. This trauma to the toenail puts you at enormous risk of developing toenail fungus.

If you had athlete’s foot or toenail fungus in the past you have to kill of all the fungus in your running shoes. The easiest way to do this is to use an ultraviolet shoes sanitizer. Ultraviolet light will actually kill all of the fungus without any odor or toxic chemicals. The other alternative is to spray the inside issues with chemical disinfectant. If you use a disinfectant spray to kill the fungus in your running shoes make sure that it dries out completely before you wear them again.

As long as you keep the fungus out of your running shoes and allow your shoes running shoes to dry completely after your workouts you should be able to prevent your toenails from turning yellow as the result of a toenail fungus infection.

No. Although you can get a nail fungus infection after you bump or bruise the toenail, it is not a good idea to treat a black and blue toenail with the laser fungal removal treatment. The black and blue discoloration of the toenail is really just a collection of dried blood under the nail plate caused by a bruised nail bed under the toenail. 

The problem is that the laser is simply very highly focuses light energy concentrated into a powerful device; powerful enough that the laser can vaporize fungus on contact. 

There is an experiment that children are often taught in elementary school about light energy and surfaces that absorb or reflect energy. Two kids each take a magnifying glass and focus sunlight into a pinpoint spot on a sheet of paper. One kid has white paper. One kid has black paper. Another kid starts a timer to see how long it takes for the concentrated beam of sunlight to generate enough heat to burn the paper.  

The black paper always absorbs more light energy than the white paper. So the black paper starts to smoke and then catch on fire much sooner that the white paper. 

When you focus a toenail laser on a black or blue bruised area of a toenail, the laser light energy can quickly heat up causing a burn of the nail bed, or even a blister under the toenail. For this reason, you would not want to treat the black and blue areas of the toenail with the laser. You should however monitor for any sign of yellowing after the bruise in the toenail is resolved.

Dr. Christopher Segler is an award-winning podiatrist who is Board Certified, American Board of Podiatric Medicine.  He makes podiatry house calls in San Francisco, Mill Valley, Oakland, Berkeley, Hillsborough, and Woodside. If you have a question about a toenail infection, you can reach him directly at 415-308-0833.