Sunburn times and elevation

Dear Tom,
"We will be vacationing in the Rocky Mountains in Colorado in early July and
camping out for several days, at an elevation of about 7,500 feet. What
effect does elevation have on sunburn times?
The Jansen Family, Chicago
Dear Jansen Family,
The atmosphere offers partial protection from ultraviolet (UV) light, the
component of sunlight that causes sunburn and other damage to our skin. It
stands to reason, therefore, that there is progressively less protection at
higher elevations.
Dermatologist Dr. Bryan Schultz (who prepares the sunburn times that appear
on this weather page) says, "UV intensity increases in an exponential
fashion as altitude increases, from 4-10 percent per 1,000 feet. With
Chicago at 620 feet, an extra 6,900 feet will increase UV over 50 percent.
Chicago's sunburn time at 1 p.m. on a clear July 5th would be close to 16
minutes, but under 10 minutes at 7,500 feet in Colorado."