Chapter 01
Four broad categories cover almost all of it, and each one calls for a different fix.
PM2.5 is the pollutant scientists worry about most. The particles are so small about 1/30 the width of a human hair, that your lungs can't filter them out. They slip into the bloodstream, reach the heart and brain, and over time raise your risk of heart disease, stroke, and lung cancer.
The ozone layer up in the atmosphere protects us. Ozone down here doesn't. It forms when sunlight reacts with car exhaust and factory emissions, and it burns the airways of anyone breathing it, worse for kids, athletes, and people with asthma.
Carbon monoxide is colorless, odorless, and pretty much invisible until it hurts you. It grabs onto your red blood cells 200× more easily than oxygen does, which is why a leaky furnace or a car left running in a garage can be fatal so quickly.
These two are the classic byproducts of burning fossil fuels. They inflame airways, trigger asthma attacks, and mix with moisture in the air to form acid rain. SO₂ is also what turns the sky over industrial cities that dull, hazy color.