-20 if not more,Celsius? The problem with pea whistles is,moving parts & if the pea were to malfunction,stick,disintegrate,etc. the chamber didn't produce enough sound to be audible.Myth or Not,An emergency is not the time one would like the chance,for this to happen,Even if it's a slim chance!Personally,I would like to get by with more than the hair of my chinny chin chin,How about you,Aaa!