Air pollution is caused by contaminants such as smoke and car exhaust. In cities, a hazy combination of gases and soot causes "smog".
Reagan said: "Approximately 80 percent of our air pollution stems from hydrocarbons released by vegetation."  This was morphed into Trees cause pollution in campaign literature and used to imply the Reagan was dishonest or unintelligent, even though (as scientists have said) he was correct.
- "It's not just cars and industry that cause air pollution," says Mr Ian Galbally, from CSIRO Atmospheric Research. "Plants release highly reactive hydrocarbons that can add significantly to photochemical smog problems. That is, smog caused by the reaction of sunlight with chemical compounds like those from industry, car exhausts — and now, as we've discovered, plants," he says. Trees cause pollution after all – Reagan vindicated