The UV Hazard During Midday Exercises, and the "Shadow Rule" for UV Protection.
The amount of UV radiation exposure increases dramatically as the sun gets higher in the sky. The exposures are most intense when the sun is more than halfway to being directly overhead. During the summer months, you should take protective measures between the hours of 10:00 a.m. and 4:00 p.m. Another way to protect yourself is the "shadow rule": the UV hazard is worst when the shadow you cast is shorter than you are tall.
Other sources of Outdoor UV Exposure
Direct sun exposure isn't the only source of UV radiation in the outdoors. You can get a sunburn even when you're in the shade -- it just takes longer. The reason is that UV radiation is scattered from the surrounding blue sky as well. As the sun reaches its highest point in the sky, the rest of the sky can contribute as much as half the UV exposure that one receives.
UV radiation can also be reflected to you from the ground. Green grass doesn't reflect much, but sand, sea foam and especially fresh snow reflect more. Fresh snow can actually double the UV hazard that you receive outdoors. As any experienced skier will tell you, it is possible to get a sunburn in winter -- that's why!
Some Prescription Drugs Increase Sensitivity to UV Radiation
Increased UV radiation damage can occur with some medications, including some oral antibiotics and non-steroid anti-inflammatory drugs. Consult your doctor or pharmacist if there is any concern.
