Jim Al-Khalili talks to botanist, Pat Wolseley about her obsession with lichen and the environmental secrets it holds. This humble and ancient organism contains a wealth of information about the quality of air we breathe. Certain species thrive on road traffic pollution: others prefer acid rain. And, for the last five years, thousands of people throughout the UK have been gathering scientific data on different lichen populations in their local area and using it to monitor air pollution.