3 out of 10 people have tattoos now. That’s according to a recent Harris Poll. But if you have a bunch of tattoos, and you spend a bunch of time working out, we have a little heads up! A recent study in the journal Medicine and Science in Sports and Exercise shows that tattoos can be potentially dangerous to exercisers.

When researchers looked at the sweat rate in tattooed versus non-tattooed skin, they found that tattoos actually blocked sweat secretion by up to 50%. And since sweating is your body's way of releasing heat, it's an essential way to cool down during a workout.

The problem is that tattoo ink is injected directly under the skin at around the same depth as your sweat glands, so it interferes with their production of sweat. Tattoo ink also affects your sodium concentration, by preventing some of the natural sodium from your sweat from being reabsorbed into the body.

The good news? You're probably fine if you've got one or two smaller tattoos because you have plenty of non-tattooed skin to compensate. But the more ink you have, the greater your risk is of overheating.

So, if you have full sleeves or your whole back is tattooed, you need to be extra cautious of heat exhaustion. And it’s even more important to stay hydrated.