Why can lichens play the role of air purity indicators?

Where there are a lot of lichens, it means clean air in this place. That is why they say that lichens can play the role of indicators of air purity. They do not like dirty air, air with various polluted emissions. They also do not withstand sulphurous air. In such air, they die, although for us it is not dangerous and we hardly feel it. By their number, people – specialists can determine air pollution and the amount of sulfur dioxide. Where they are not, the air is much more polluted than in those places where they are.