Air Pollution: Primary Pollutants

Air pollution is dominated by five classes of primary pollutant, each with a distinct source and a

distinct mechanism of harm.

Carbon monoxide ($\text{CO}$) forms whenever a carbon-based fuel burns with an insufficient

supply of oxygen -- incomplete combustion:

Vehicle engines, especially when poorly tuned, and indoor stoves are major sources. $\text{CO}$ is

dangerous because it binds to the iron atom in haemoglobin roughly 200 times more strongly than

oxygen does, forming carboxyhaemoglobin. This blocks haemoglobin's normal job of carrying oxygen to

tissues, so even a modest concentration of $\text{CO}$ in inhaled air can starve the body of oxygen

long before the gas itself would otherwise be considered dangerous.

Sulfur dioxide ($\text{SO}_2$) is released when fuels containing sulfur impurities -- coal and

diesel in particular -- are burnt:

$\text{SO}_2$ directly irritates the eyes and respiratory tract, and it is also the starting point

for acid rain (covered in the next section), since it slowly oxidises in air and dissolves in

atmospheric moisture to form sulfuric acid.

Oxides of nitrogen ($\text{NO}_x$), chiefly $\text{NO}$ and $\text{NO}_2$, form when the very

high temperatures inside an internal-combustion engine or furnace are enough to force normally

unreactive atmospheric nitrogen and oxygen to combine:

Besides irritating the lungs directly, $\text{NO}_x$ is the key ingredient that drives

photochemical smog (Section 13.5).

Unburnt hydrocarbons escape from engines and fuel systems whenever combustion is incomplete or

fuel evaporates before it burns. On their own they are a nuisance and some are carcinogenic; their

larger role is as the second essential ingredient of photochemical smog, reacting with

$\text{NO}_x$ under sunlight. …