Recently, a schoolteacher in France lost his teaching job after parents complained their kids were scared of him.

The reason? The 35-year-old man has every inch of his body (even his tongue) covered in tattoos. He'd spent at least 460 hours getting inked.

Tattoo enthusiasts may not be aware of it, but such extreme decorations pose health risks — from allergic reactions to cancer caused by the black ink and heart disease and heart failure caused by the cadmium in red ink.

Now researchers have identified a new hazard: The ink can keep sweat glands from functioning, leading to chronic skin irritation, heat cramps, and heatstroke.

A study published in the Journal of Applied Physiology evaluated the amount of perspiration generated by people with tattoos. It turns out that when the body sends a message to sweat glands under inked skin to sweat, the glands just don't.

In short, "tattooing functionally damages secretion mechanisms," say the researchers.

Before getting a tattoo, consider the risks and take precautions.

• Infection? There's no way to tell if the ink is safe (some, says the Food and Drug Administration, is better suited to painting cars), and bacteria and other pathogenic materials can end up in ink. If the shop doesn't seem clean or the tattoos are deeply discounted, leave immediately.

• Check that the practitioner is using personal protective equipment, and that needles and ink come from sealed containers.

• Avoid blanketing large areas of the body with ink. Less is more.

• Do research before making your skin a canvas.