"Tis the season for the flu, and if the regular strain wasn’t bad enough, we have a new, porcine terror to contend with. These brightly patterned medical face masks, however, may ward off H1N1 and its brethren simply by virtue of being stylish—looking like Wacko Jacko in his latter days notwithstanding. Designed by Marjan Kooroshnia, a Swedish textile-design student, these face masks have a bonus feature: They’re printed with thermochromic ink that changes color with any uptick in breathing temperature." (source)
If you've ever experimented with thermochromatic inks, I am skeptical that this concept will even work as the variations of temperature from your breath might be too small. Some of the inks are fairly sensitive to heat so the pattern might change no matter what temperature your breath is. Regardless, it's a nice concept.