Our lungs are amazing things. Impossibly delicate structures of tissue capable of carrying both air and blood, allowing vapor exchange between the two. In structure, they resemble the branches of an upside-down tree, with the uncountable alveoli, the points of exchange, as the leaves.

They’re also a dead-end for anything that should not have been breathed in. Any solid matter which gets into the lungs has almost no chance of ever leaving them again.

Most of us have seen graphic comparisons between the lungs of people who do and do not smoke – we’d see similar things if we looked at the lungs of coal miners, charcoal makers, or those who live in very, very polluted atmospheres. When we breathe in particulate matter, be it smoke, grit, tar, or any other solid, the lungs trap it, coat it in biofilm, and do their best to ignore it until they can’t anymore.

According to the EPA, the safe level of PM2.5, or airborne particulate matter, to be breathing is 35 micrograms per cubic meter. Scaling that up, that’s approximately equivalent to a teaspoon of dust spread out through a cube 20 meters to a side.

Recently, researchers looked at subway systems in major U.S. cities, and found alarming levels of PM2.5. The New York subway system, for instance, was measured at 251 micrograms per cubic meter, and many of those particles were heavy metals. Worst of the worst was the Christopher Street station in Manhattan, which registered an incredibly dangerous 1,499 micrograms per cubic meter.

The researchers said this level of particulate pollutants compares to wildfires or active building demolition. It includes everything from exhaust from diesel locomotives left over from decades past, to organic contaminates from hundreds of thousands of commuters, to the mummified remains of dead rodents. Almost makes you want to keep on wearing a mask long after the pandemic passes.

Photo: The Christopher Street Station in New York City is the most polluted of all the stations the researchers visited. Credit: Shutterstock