They are toxic. Our lungs have defence mechanisms designed to prevent us inhaling dirt and dust. The mucocilia escalator in your lungs, plus an army of white blood cells, work hard to keep your lungs clean.

The problem is that you’re only equipped to deal with a very small amount of dirt and dust. And even then, you get damage. This is how your lungs wear out over the course of your life. Inhaling smoke, city air, water vapour - it all wears your lungs out faster.