Being fair-skinned, my time outdoors is usually done from inside a thin candy shell of sunscreen and sweat. Even that rarely protects me from sunburns, as my skin has a charming tendency to withstand sunlight for hours, then go from rosy to rock lobster in minutes. It's all in the amount of actual UV I'm getting, I guess, but it's impossible to tell how much radiation I'm absorbing when even a 100 watt bulb is enough to make me flush and reach for the lemonade.
I could buy this "Portable UV Sensor" from Oregon Scientific which monitors incoming UV rays and can calculate exposure time based on the SPF of my sunblock. (Provided they have a setting for "sludge.") But I won't because it's one more thing to haul around in the outdoors. And the last thing I need is an alarm confirming mid-hike that yes, I am indeed going to regret leaving the house.
The Portable UV Sensor sells for around $20. Perfect for lifeguards!