This is not a post about obesity. Despite the fact that I have quit exercising on days where the AQI is high.
Instead, it’s about trying to weigh air pollutants.
Two things occurred to me.
First, I have to take air filter ratings as a matter of faith. I can see that the filters get dirty over time, but in terms of the quantity of material extracted, I have to rely on the MERV rating or similar.
Second, there’s so much crap in the air these days that, if my air filters are catching most of it, I ought to be able to detect my air filters getting heavier over time. In fact, I know that’s part of the testing standards for air filters. But I always figured you needed sensitive lab equipment to detect the weight gain.
The only question is, could I detect that weight gain using a simple kitchen scale calibrated in grams? After all, I was able to use a kitchen scale to see how large the losses were from my old Blitz gas can (Post #1773, Why do all gas cans suck?)
Surprisingly, the answer to that is yes, easily. At least, that’s what the theory says. These filters ought to be extracting several grams of material per day. I ought to be able to weigh my air filters on a daily basis and infer how much particulate of all sizes they have extracted from the air.
A little back-of-the-envelope
Assumptions:
- Let’s say we have an AQI of 100 for particulates up to 10 microns in size (PM 10). That would translate to about 85 micrograms (millionths of a gram) of PM 10 per cubic meter of air.
- A 3M Filtrete 1900 ought to be able to pick up at least 80% of those in a single pass.
- A box fan ought to move at least 750 cubic feet of air per minute (CFM).
The rest is just arithmetic.
The bottom line is that if I only consider PM 10 — no hair, fibers, etc. — running my filter-on-box-fan setup outside, for a day, should result in a two gram weight gain for the filter.
If I broaden that to total particulates (everything up to about 100 microns, not 10 microns), that ought to give me a 3-gram-a-day increase. (Because that’s the typical ration of total particulates to PM 10, as I discovered in a recent post on air pollution).
Pretty sure I haven’t messed up the arithmetic, but something about this doesn’t quite square with my experience. Three grams a day doesn’t sound like much, but that’s a rate of about three ounces per month. This, for a filter that only weighs about 10 ounces when new. If these accumulated that much weight, that quickly, you’d think I’d have noticed it when replacing a dirty three-month-old filter.
On the other hand, a) the air is rarely this dirty, b) I’m filtering indoor air, which I surely hope is not as dirty as outdoor, and c) I’ve never thought to compare the weights of clean and dirty air filters.
So it doesn’t seem quite plausible to me, but it’s not as if I’ve ever tried testing it.
I’m not quite sure where I’m going with this. If I want to test the particulate levels indoors, I can do that with meters that seem to be priced lower than the cost of another fan and filter. So, in theory, I can get a direct measurement of how effective my air cleaners are, without trying to infer it from weighing air filters.
In the end, I guess I’m just amazed that there’s so much fine particulate in the air that I ought to be able to watch my air filters getting heavier, day by day. Kind of like watching paint dry, but slower.
