Beware of covering your kids head to toe in SPF-50 sunscreen. It could cause them to develop a painful condition straight from the 17th century: Rickets! Because it blocks the body’s production of Vitamin D.

And an extreme Vitamin D deficiency can cause the bones to become soft, painful, weak and deformed. Kids can even develop bowed legs, a curved spine, and thick ankles, wrists, and knees. The disease was common in Northern climates until the 1940s, when milk began being fortified with vitamin D. After that, it was almost unheard of. 

But recently there’s been a surge in children’s diagnoses. That’s because kids’ growing bodies need Vitamin D from the sun to properly develop. But lately, there’s been an “avoid the sun at all costs” message spread by a lot of health professionals. A lot of kids barely go outside these days. And when they do, they’re often slathered in the highest SPF sunblock available. 

But experts warn that high SPF sunblock is so strong that it’s preventing their growing bodies from producing any Vitamin D. Rickets can be treated through diet and Vitamin D supplements. But it can take years to cure.

So, doctors suggest the best way of staying safe from the sun, and avoiding rickets, is to eat a Vitamin D-packed diet. Including dairy products and cereal. And spend 15 minutes a day outside sunblock-free, then apply a lower level sunblock, like SPF 30.