Well, there's actually no evidence that exposure to cold and dry weather by itself might cause someone to catch a cold. Cold is transmitted from person to person, and cold dry weather just make people more likely to be close to each other, and droplets of coughing/sneezing people travel further in the air. But you still need someone with a cold around. So if nobody in the team currently has a cold, Tuuri is unlikely to spontaneously get one from just working outside.