It's cattle farts, straws, and bottled water.
Actually, it's cattle belches that emit methane ... comes out their mouths and not their butts ...
And if you don't like my other source
Although air quality has improved significantly since the 1990s, a new study claims that pollution still causes lung cancer, heart attacks and strokes that kill more than 30,000 Americans a year.
Researchers examined the concentration of fine pollution particles across the country from 1999 to 2015. The particles, which according to the EPA are 30 times smaller than the width of a human hair, come from coal-fired power plants, cars and other sources.
Reducing air pollution could save thousands of lives, researchers say