I live at 5100 feet. Camp, fish, hunt, hike, etc at 7000 feet plus, and they always work for me.
I hunt because......
Bugs are bad in OR too this year, especially ticks. Sometimes it feels more like we're a couple of chimps out there picking things off each other as they try to burrow in UGH! ThermaCell's are the biz - works great and that's coming from someone who suffers bad reactions to mosquito bites. Rarely get bitten with a Thermacell. Other good thing I've found for ticks is Under Armour compression heatgear/coldgear. Has to be the compression fit though to stop them getting through.
I was just thinking I need me a ThermaCell. I read that they're not designed to run at altitudes above 4,500 feet.
Arise... Kill, Eat! - Acts 10:13
@Elkoholic307 - you could try one of these instead at high altitudes as they are designed for it. Not so easy when you are out and about, but great at camp. http://www.thermacell.com/mosquito-r...ern-2006-model