Wood burners produce 40% of the small particular air pollution in urban areas. Only about eight % of people use them, and virtually all of them have other sources of heating. A report this year found that it's cutting the lives short of billions of people by up to six years. But another form of pollution where there's still quite a lot of research on going into its health effects is microplastics. We've polluted the entire planet, and we don't really know what it does to people.