Vulnerability to wildfire smoke is in our genes, thanks to an evolutionary quirk

When the sky becomes orange and hazy from wildfire smoke , modern humans are at greater risk than our earlier ancestors would have been.

This is because mutations that occurred tens of thousands of years ago made humans exquisitely sensitive to some types of air pollution.

Wildfire smoke includes two dangerous components : volatile chemicals, which include the things you can smell, and small particles, which you can’t .

Both these components cause a lot of inflammation when we breathe them in and can trigger asthma attacks, and exacerbate chronic obstructive pulmonary disease (COPD) and heart attacks .

We don’t have any immune defences to stop the volatile chemicals, but we do have a few layers of protection from the small particles.

Some of these particles get trapped in the mucus that lines our nose, mouth and lungs and are cleared out as the mucus moves up and out of our lungs and is swallowed. (Count how many times you swallow while you read this article — you probably do it so automatically you aren’t aware of just how often you are helping clear the gunk out of your lungs).

Macrophages and the scourge of tuberculosis Particles that aren’t cleared out by the mucus are generally eaten up by immune cells called macrophages .

Macrophages are voracious eaters (their name means “big eater”) of things that are about the size, shape and electrostatic charge of bacteria.

Unfortunately, an evolutionary quirk means that macrophages found in humans are less good at eating the particles found in air pollution and wildfire smoke than the macrophages found in other animals .

The size of particle that causes us the most grief is about the same size as a bacterium (about 2.5 microns).

One particular type of bacteria called Mycobacterium tuberculosis is a particular problem for our macrophages.

M. tuberculosis causes tuberculosis, or TB, and has been a scourge of humanity since even before we were humans .

Most bacteria do everything they can to stay away from macrophages because once they are eaten they are destroyed, but M. tuberculosis can’t survive unless it’s inside a macrophage.

Once it gets into a macrophage, there’s a macrophage-pathogen quick draw and if the macrophage doesn’t win and kill it immediately, M. tuberculosis will go on to cause an infection.