The passive heat exchanger I made in the early 80ies to be able to bicycle to my work also when it was cold, like -30 C (-22 F) in winter, when the cold air is quite tough for your lungs. I didn't like to take the car because it was just a few km away and bicycling was more refreshing.
This heat exchanger is based upon the simple principle that warm air from your lungs heats the cupper fibers which will then warm up the cold air before entering into the lungs.
Heat exchangers similar to this one, but of plastic and using Aluminium instead of cupper, occured on the market first around 1986 as I remember (again a patented design...). I never bought the commercial one because I considered my was working better, and I already had it.
Problem: even if you can easy think up something like this, which most people who knows some basic physics can, not all people are able to build them, because they need to have some talent to improvise and be handy.