Each indoor space is unique, with its own sources of pollution and rate of exchange with outside air. Pollutants can build up indoors so Outside air is usually cleaner than indoor air. A great way to stop pollutants from building up is to remove them with an air purifier that filters out toxic particles and gasses, but their total impact is never completely clear because air pollution is constantly coming back. At least one study has found that after about one hour, the incoming pollutants and the pollutants removed reach an equilibrium, but exactly what the equilibrium level is or if it’s healthy requires a closer look.
Most air purifiers come along with at least some instruction of how quickly they move air, but speed alone doesn’t tell us how quickly an air purifier takes effect or if different pollutants are cleaned at different rates. Let’s take a look at how indoor sources of pollution work and how quickly we might expect an air purifier to handle them.
What is the Air Exchange Rate?
The air exchange rate, or ACH, is a measure of how quickly outside air comes into the room and pushes out indoor air. The indoor air is never completely pushed out, some will linger in stagnant corners or other areas, so the ACH is more a measure of how much outdoor air is coming in rather than indoor air going out. When the ACH is too low, carbon dioxide and any other pollutants present could reach problematic concentrations.
Even when the doors and windows are closed, outside air is creeping in and pushing out the air that is indoors. The actual rate is very difficult to determine without specialized equipment, but organizations like ASHRAE set standards for occupied rooms. They recommend an ACH of about 0.35 per hour, or enough outdoor air to displace 35% of the room’s indoor air, but no less than 15 cubic feet per minute per person. That means an ACH of 0.9 is required for one person in a 10x10x10 room, twice that for 2 people. Almost no one has a room that automatically adjusts ACH depending on occupancy, so instead we usually open the window when it gets stuffy.
When there’s no wind outside and the indoor temperature and outdoor temperatures are equal, a 5 square foot window in a 10x10x10 foot room will bring in a full room-sized volume about every 20 minutes with an ACH of 3.5. If there is a 20°F difference between indoors and out, the rate can be as high as every 5 minutes or an ACH of 8. So opening a window that isn’t really small should easily keep the air fresh.
Air purifiers also have an ACH, but they don’t do the same thing as outdoor ventilation because they can’t remove carbon dioxide and could be so slow with other pollutants as to be ineffective. An air purifier’s ACH should be higher than the recommended outdoor air ACH because the air from a purifier isn’t as clean as outdoor air.
How quickly do pollutants build up?
In addition to knowing how much clean air is coming in, it’s also great to know how quickly the air is getting dirty. This is impossible to know with perfect accuracy, but some sources of pollution will get into the air in large concentrations in a short amount of time.
Fire, smoke, and sprays build up in minutes
One of the biggest indoor air quality problems is cooking. When temperatures get much above boiling, unhealthy particles and gasses will start to fill the air. Activities like frying can worsen air quality well beyond safe limits within minutes. In addition to a range hood and any other ventilation, you can add another layer of protection by using an air purifier while cooking to reduce the speed at which the pollutants build up and to remove them after cooking is done.
Cleaning products with bleach and artificial scents can also be air quality hazards that show up quickly, particularly if they are sprayed through the air. Within a few minutes of using harsh cleaning products, indoor air quality can be as bad as if a car were running in the room. Air purifiers are also great in these circumstances and can lower pollutant concentrations to some degree, but won’t rival venting through the window.
Off-gassing and odors are slower
Some building materials and furniture can slowly release chemicals from manufacturing. The degree to which these chemicals, often referred to as VOCs or volatile organic compounds, are released from different materials varies widely, but is generally slow. This presents an issue because unlike the potent scents of smoke or cleaning chemicals, the VOCs may not have a noticeable odor as they build up. Air purifiers that remove or destroy VOCs are particularly effective for this kind of off-gassing because they can prevent the accumulation over time.
Along similar lines, stinky smells like pets, people, or just mustiness takes some time to reach concentrations that are noticeable or problematic. Air purifiers that can remove gasses are also good for these issues because they can keep smells below the threshold where they could be noticed.
Allergy triggers are fast and sticky
Tiny proteins on the surfaces of known sneeze causes like pollen, mold spores, and dust trigger allergies. As any allergy sufferer knows, a pollen-producer doesn’t have to be in the house long before symptoms start up. What’s worse is that allergy triggers tend to stick around and can be kicked back up into the air to trigger a reaction for a long time. They’re usually on bigger particles that settle to the ground, but an air purifier can reduce the very light bits that remain airborne for extended periods. If you’re an allergy sufferer be sure to wipe and dust frequently to minimize reservoirs of allergy triggers.
How fast is an air purifier?
There are a few different ways to measure an air purifier’s performance because different pollutants are trapped at different rates. Whole pollen grains are relatively large and it’s very unlikely that the air coming out of any purifier will have any whole pollen grains or mold spores, but small fragments or individual molecules of gas can penetrate many filters.
The speed of an air purifier depends a lot on how fast it can move air, but also how well its particular configuration of filters captures pollutants. If it stops 90% of particles but 50% of gas molecules, then it technically has different speeds for particles and gasses.
Most consumer air purifiers can move somewhere in the neighborhood of 75 cubic feet of air per minute (cfm) for smaller purifiers, and up to around 350 cfm for a larger purifier. What that means for ACH depends on the size of the room. A small 250-square foot room (assuming 10 foot ceilings) would get about 2 ACH from a 75 cfm purifier, while a larger 350 cfm purifier would be better for 1,000 square feet. Let’s take a closer look at some performance numbers of Molekule purifiers on specific pollutants.
How fast can an air purifier remove particles?
When we tested Air Mini+ on smoke particle removal, we found it was able to remove smoke very quickly. Judging from the test linked in this blog that was done in a 10x10x10 foot room which is a smallish bedroom, the amount of smoke that Air Mini+ removes is about the same as having ventilation with an ACH of 2 (if smoke was the only pollutant), or putting out an amount of cleaned air equal to 2 smallish bedrooms per hour. In the same space, Air Pro was predictably much faster, getting to the equivalent of an ACH of 6 for the same sized room. No purifier can remove all particles, but when used in the right sized room, the air should breathe a lot cleaner after no more than 30 minutes.
How fast can an air purifier remove gasses?
This answer is much more complex because different gasses have different physical properties that can make them harder or easier to capture. Limonene, which is one of the molecules that make up the smell of citrus, was captured by our purifiers about half as quickly as the smoke in the previous section. However limonene’s chemistry makes it easy to capture with activated carbon, which our purifiers have in their PECO-HEPA filters.
The purifiers were also tested on the pollutant formaldehyde, which is both more toxic and has chemistry that makes it more difficult to capture than limonene. Our purifiers don’t just capture formaldehyde with carbon, they also destroy it with PECO, but still remove much slower than limonene, only a fifth as much per hour. Formaldehyde is a VOC from off-gas and should be building up slowly, so using an air purifier can help to prevent build-up, but opening a window is also a great method.