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The environmental guidelines for the iPhone 5 specify humidity between 5% and 95%. That’s a range from essentially desert dry to subtropics. However, and this is a big however, Apple specifies up to 95% NON-CONDENSING humidity. This means if your iPhone is cold from sitting in front of an air conditioner, and you go outside in 95% humidity, it could easily be below the dewpoint and moisture could condense, the same as a cold glass does on a hot summer day. This could potentially trigger the Liquid Contact Indicator (LCI) and void your iPhone’s warranty even though you did not drop your iPhone in liquid or spill anything on it.
Apple has been sued over denied warranty claims due to false LCI readings in the past. I am not certain whether the design has been improved at this point, but a few years ago there was strong evidence that the LCIs were not 100% reliable. http://forum.hardmac.com/index.php?showtopic=7725
By the way, you can look and see for yourself if the LCI has been activated. It will turn red when exposed to liquid, and is viewable on iPhones without opening the case. On the iPhone 5 it is on the middle of the right side of the phone. This link will show where the indicator is on iPods and iPhones. http://support.apple.com/kb/HT3302