As large parts of the West are engulfed in wildfires and smoke blankets the area, I’ve started to see new arguments for and against mask wearing. Official guidelines from the CDC indicate that most masks are unable to filter out particles as small as those causing smoke, and I’ve seen a lot of angry claims that if masks cannot filter out smoke, they surely must not be effective against viruses such as SARS-CoV-2.
This question is ultimately one of size and scale: it requires knowing the relative sizes of viruses, the compounds in smoke that are harmful to lungs, and the sizes of pores in masks. In fact, I’ve seen many other misconceptions also related to scale: that masks cut off oxygen supply while allowing viruses through, that carbon dioxide can be trapped in the mask, that if smells can get through a mask, so can a virus.
Let’s take a look at scientific scale and the relative sizes of these molecules and larger particles.
Things we breathe: 2 Å on average
The air we breathe is made up of around 78% nitrogen, 21% oxygen, and trace amounts of other gasses and water vapor. Gaseous oxygen (O2), which is required for our body to carry out the fundamental process of turning the things we eat into energy for our body, is made up of two oxygen atoms. For our purposes, imagine two Skittles glued together. Gaseous nitrogen (N2) is also made up of two identical atoms, but this time nitrogen atoms instead of oxygen. In other words, air is mostly composed of a two-atom molecule (molecules contain multiple atoms). That puts their size at roughly 2 angstroms: something so small, it is perhaps difficult to imagine. An angstrom is approximately the size of a single atom. As a point of reference, atoms are so small that it is estimated there are more atoms in a single grain of sand than stars in the known universe. (For a better idea of scale and how small atoms are, check out https://scaleofuniverse.com/powers-of-ten/.)
Things we smell: 10 to 100 Å on average
The smells that we sense are caused by specific molecules (collections of atoms) which set off very small receptors in our noses. These molecules are often called odorants or aroma molecules (read more here). I remember the organic chemistry lab at Great Basin College used to produce the molecule responsible for the scent and taste of banana: isoamyl acetate. It was pungent, filling not only the lab but pretty much the entire building with the smell. This molecule contains 7 carbon atoms, 14 hydrogen atoms, and two oxygen atoms for a grand total of 23 atoms. Using the average length between two atoms bonded to each other, we get an estimated size of 9 angstroms. In other words, the molecules responsible for odors are only around 5 times larger than the molecules we breathe.
Smoke from wildfires is a complex mixture. It contains many gasses that are irritating or even toxic to the tissue in the throat and lungs, including carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2), and sulfur dioxide (SO2). These gasses are only around 2-3 Å in size. In contrast, soot and other fine particles from burning trees, brush, and other materials are closer to 25,000 Å. These particles primarily harm the lungs and airways through physical irritation as they rub against the tissue. As anyone who has been next to a campfire or wildfire is aware, soot particles can be much larger and even visible to the eye (usually called ash).
Viruses: 1,000 Å average
Viruses contain around 4 million atoms and are, on average, one thousand times larger than oxygen and nitrogen molecules. They vary in both size and shape, from the roughly spherical rhinovirus (the cause of the common cold; rhino means nose) of 300 Å in diameter, to the narrow and elongated Ebola virus which can be nearly 10,000 Å long! Viruses such as the influenza virus (the cause of the common flu) and SARS-CoV-2 are roughly spherical and approximately 1,200 Å in diameter.
N95 mask pores: 1,000-3,000 Å
These masks are widely considered the best at protecting the wearer from bacteria and viruses, and, in the current pandemic, are generally reserved for frontline healthcare works most often exposed to SARS-CoV-2. The 95 in the name indicates how effective the masks are: filtering out approximately 95% of particles on the range of 1000 to 3000 Å. This means that individual viral particles are just small enough to be partially filtered out by these masks, although we’ll see below that viruses usually aren’t spread as single particles but rather in larger droplets. In contrast, most surgical and cloth masks have much larger pores (10,000 to 100,000 Å).
Droplets (from coughs and sneezes): 500,000 to 1,000,000 Å
Perhaps one of the biggest misconceptions about viral spread is that for a mask to be effective, it must filter out all particles that are the size of a single virus. If this was the case, N95 masks would barely be sufficient to stop viral transmission, and cloth and surgical masks would be an absolute joke. Fortunately for everyone living through this pandemic, this is not the case. Viruses are very rarely spread as individual viruses floating through the air (called aerosols). Most of them, including SARS-CoV-2, are spread primarily as droplets: small specks of moisture and water that you expel when you breathe, talk, sneeze, or cough. In fact, the majority of these droplets are large enough that even a tissue or handkerchief is able to stop them, as anyone who has ever been sick with the cold knows (and in fact, the virus responsible for the cold is 3x smaller than the SARS-CoV-2 virus!). In addition, individual viral particles are likely still slowed down by masks, similar to the way traffic slows down when lanes merge on the freeway. This is in line with multiple studies that have found surgical and cloth masks greatly reduce how far someone sick expels the virus into their surroundings through the nose and mouth.
The scale of objects we’ve covered in respect to masks is enormous: from 2 to one million angstroms! Most of the gasses we breathe and exhale (oxygen, carbon dioxide, and smoke components such as carbon monoxide and nitrogen dioxide) are only around 2 Å. The organic molecules we perceive as scent are not much larger, around 10 to 100 Å. In contrast, viruses are around 1000 Å, and droplets (the main way viruses like SARS-CoV-2 spread from person to person) are closer in size to 1 million Å (often visible to the unaided eye). With these scales considered, N95 masks will be minimally effective against individual viruses and cloth, surgical, and N95 masks will all be very effective against virus-containing droplets. In contrast, the gases we inhale and exhale and most scent-causing molecules will readily go through masks.
Hannah Margolis is a postbaccalaureate researcher at the National Institutes of Health with a degree in biochemistry from Dartmouth College. She can be reached at email@example.com.