What is the relationship between air pollution and asthma?

When it comes to effects on asthma, air pollution as a whole can be sub-categorized into...

Gaseous pollutants:

• Ozone (#O_3#): from reaction with nitrogen oxides and volatile organic compounds (VOCs)
• Nitrogen dioxide (#NO_2#): from burning of fossil fuels at high temperature in cars, power plants, etc
• Sulphur dioxide (#SO_2#): also from fossil fuel combustion in power plants, industrial facilities, trains, ships, etc.

Particulate matter (PM):

• PM 10 aka " coarse particles " (2.5 to 10 micrometers in diameter): dust, pollen, mold, smoke, dirt, dust
• PM 2.5 aka " fine particles " (<2.5 micrometers in diameter): toxic organic compounds, heavy metals made by vehicle combustion, burning things, smelting and processing metals

And a general mixture of gaseous + PM pollutants produced by motor vehicles, the main contributor to urban outdoor pollution. This mixture is commonly referred to as Traffic-Related Air Pollution ("TRAP") .

Here's a nice visual of sources of air pollutants:

When these various air pollutants are inhaled, they irritate the linings of your airway from your nose and throat all the way down to the airsac endings (alveoli) inside your lungs.

The exact mechanisms of irritation and your body's reactions depend on the type of pollutant. In general they cause oxidative stress, airway inflammation and hyper-responsiveness in the airway. When you have asthma, this leads to:

• increased mucus production,
• thickened walls,
• and tightening airway muscles

all of which causes the airway to narrow which makes it harder to breathe (get air in and out of your lungs) - an asthma attack.

What happens to your airway during an asthma attack:

There's also growing evidence that more exposure to air pollution, especially of TRAP (from motor vehicles), is linked to increases in new-onset asthma, especially in children.

Multiple prospective (forward-looking) studies that followed children and adults who did not have asthma at the start of the studies have found increased incidence (rate of new asthma cases developed) that was associated with more exposure to TRAP.

The exact mechanisms of how these air pollutants may cause development of asthma is not clear yet, although there are a framework of 4 likely main mechanisms under study :

More reading:
2014 Series Article in the Lancet: "Outdoor air pollution and asthma"