Category: Environment

Post #1794: Why filtering forest fire soot is not the same as filtering aerosol droplets.

 

I learned a lot about air filtration during the recent pandemic.  At some point, I wrote down and compared all the different standards used for air filtration.  For example, what does HEPA actually mean, and how does it compare to the various members of the MERV clan?  And how do all of those relate to N95? (Post 593, April 1, 2020).

The problem-du-jour isn’t about filtering tiny little viruses out of the air.  Instead, it’s about filtering tiny little soot particles out of the air.  In the U.S. Northeast, we’re now in an era of Canadian wildfires and the resulting air pollution alerts.

Nicely enough, I get to re-use what I think I already know about air filtration.  The knowledge that applied to filtering aerosol droplets (droplets less than 5 microns in size) applies equally to filtering fine particulates such as the soot from wildfires.  This soot typically falls into the PM 2.5 air pollution category, that is, any particulate matter in the air less than 2.5 microns in size.  (For comparison, a human hair is typically around 70 microns thick.)

Except that there is one key difference between filtering the air for pandemic purposes, and filtering the air for forest fire purposes:  Outdoor air is no longer our friend.  In fact, outdoor air is now the enemy.

When filtering viruses, outdoor air could be assumed to be clean.  The likely concentration of virus droplets in outdoor air was typically negligible.  Disease transmission in outdoor settings was virtually unheard-of.  You only had to worry about the “pollution” that you generated inside the indoor space.

But for forest fires, the entire problem IS the outdoor air.  In some sense, the entire battle to keep indoor air breathable is about dealing with the dirty air outside.

To crystalize this, recall that one standard suggestion for improving the COVID safety of (e.g.) schools and other spaces was to open the windows.  Perfectly rational if you’re dodging COVID.  Not so smart if you’re trying to avoid forest fire smoke.  So, from the get-go, you now see that dealing with forest fire smoke is a completely different engineering challenge from minimizing aerosolized COVID. 

Why does this matter?  Outside air is always leaking into indoor spaces.  For the virus filtering, that was a good thing.  Now it’s not, and you need to strategize your air filtration accordingly.  Why?  Because outdoor air doesn’t just leak into indoor spaces, it typically pours in.  The minimum standard for houses, from a health standpoint, is that outdoor air should replace indoor air at least once every three hours (reference).  But a typical well-constructed older home, in good shape (storm windows, caulked) would exchange all the indoor air with outdoor air once per hour (random reference).  Leakier construction (no storm windows, caulk missing) would experience air exchanges more rapid than that.

Whatever air filtration setup you use, you now need to account for that.

My particular problem.

My daughter lives in New York City.  Shown above, New York just had a bout of extremely bad air quality, due to Canadian forest fires.  At the peak, PM 2.5 (particular matter 2.5 microns or smaller) reached 460 micrograms per cubic meter.  This is way beyond what the EPA considers hazardous, and is maybe 10 to 20 times the typical value for that area.

Given that those fires continue to burn, I’m guessing this isn’t a one-and-done.

So I wanted to get her an effective air purifier for her apartment.  (And even if forest fire soot does not return, a New York City apartment would probably benefit from having an air purifier).

My options were to go with a redneck air purifier (20″ box fan, and a high-quality electrostatic air filter), or to buy a purpose-made room-sized HEPA air purifier.   The redneck air purifier is a variation on the “Corsi box”, a D-I-Y air purifier that was promoted as an easy fix for indoor air filtration during the pandemic.

You might normally say that HEPA must be better, because it’s a higher filtration standard.  In theory, HEPA filters must remove 99.97% of fine particulates in a single pass (based on the Wikipedia entry for HEPA).  In practice, I think 99.5% is more typically advertised.  Whereas a high-end air filter (in this case, Filtrete 1900) only removes about 65% of fine particulates in a single pass.  Seems like the case for HEPA is a no-brainer.

And if I were trying to filter the air in a hermetically-sealed box, HEPA probably is the better choice.  The only drawback to HEPA is the back-pressure of doing that high level of filtration limits air flow, for a given power input.  HEPA units advertised as “room-sized” air cleaners typically filter just a few hundred cubic feet of air per minute.  By contrast, the entire selling point of the 3M Filtrete electrostatic filters is that they achieve a reasonable degree of fine-particle filtering with minimal back pressure.  With a box fan on low, I can push 1000 cubic feet of air per minute, through a Filtrete filter.

My gut tells me that, for older construction, with a lot of air infiltration, the cheap setup (box fan and filter) is better than the equivalent purpose-built HEPA air filtration unit. A typical room-sized HEPA unit isn’t going to be able to “keep up with” the inflow of dirty outside air.  Or, at least, not as well as the high-air-flow fan-and-filter setup.  If a lot of air is flowing into the room, my gut tells me that that HEPA (high-filtration/low-volume) actually does a worse job than box-fan-and-filter (low-filtration/high-volume).

Many mathematical paths to the sea.

At some level, I realize that I’m trying to solve a classic calculus problem.  Typically, it’s a water tank with inputs and outputs.  Here, it’s a room with leaks and a filter.  There must be a classic closed-form solution that would tell me the final concentration of particulates in the air. Just plug in the parameters, and view the output.

Somewhere along the line, I have lost the ability to cast a problem such as this into that classic format.  No problem.   Much of what used to require actual intelligence and insight can now be done with brute-force computing power.

In this case, all I need to do is the simple numerical simulation, in a spreadsheet.  Start with a room full of dirty air, with a known rate of infiltration of outside air.  Turn on an air cleaner with known properties.  And just do the accounting, minute-by-minute.  Air in, air cleaned, air out.  And track the resulting concentration of pollutants in the air.

Does anybody ever care about the details of methodology?

Answer:  Only if they disagree with the results.  By contrast, if I end up saying something you agree with, you won’t care how I arrived at that.  That’s just human nature.  People just want to say Amen and move on.

My redneck air purifier consists of a box fan pushing 1000 CFM (on low), through a 3M Filtrete (r) filter.  The Filtrete is a 3M “1900”, rated at MERV 13.  It grabs 65% of the PM 2.5-sized particles with each pass, and about 95% of PM 10-sized particles.  For this simulation, I’m only tracking PM 2.5.

B

The only practical detail you should care about is that if you do this — a single filter stuck on the back of a fan — you must use a low-back-pressure electrostatic filter.  Otherwise, if you want to use el-cheapo MERV 13 filters, you need to go to the trouble of actually constructing a literal Corsi box, using four filters taped together.  That’s because with cheap filters, you need all that surface area to avoid the high back pressure that would starve the fan of air.  See this reference for Corsi box.  Even with that, my take on it is that it provides slower air movement than using a single Filtrete 1900 placed on the back of a fan.

Below is a literal Corsi box, via Wikipedia.  If you use cheap filters, go that route.

By contrast,  my theoretical HEPA filter pushes 100 CFM through a filter that grabs (say) 99.5% of PM 2.5 particles.  (Separately, I’ll show the results for a filter pushing higher rates of air flow).

The room is 20′ x 20′, with an 8′ ceiling, and has one full air exchange per hour, typical for sound older construction.  That is, enough outside air enter the room through various leaks and cracks that it would be enough to replace the air in the room once per hour.

The outside air is at 450 micrograms per cubic meter of PM 2.5, the peak of the air pollution in the New York City area.

Based on this, I’ve written the spreadsheet that does the accounting.  Air in, air purified, air out.

Results.

The results of my Excel-based numerical simulation validate what my gut was telling me.  Due to the high rate of air infiltration typical in older construction, filtering the air rapidly is far more important that filtering the air extremely well.

On the left, you see the results for my redneck, box-fan-plus-Filtrete air filtration unit.  It passes 1000 cubic feet per minute, but only filters out 65% of the finest (PM 2.5) particles.  On the right, you see the results for a slower HEPA unit.  It passes one-tenth of the air per minute, but it filters it more than 10x better.

The equilibrium level of particulates in the room is vastly lower with the high-volume, lower-efficiency filter (left graph above).  Why?  Because the slow pace of the HEPA filter (right graph) can’t keep up with the level of outside-air infiltration that is typical in older construction.

To get the HEPA filter to work almost as well as the simple Filtrete (r) 1900 plus box fan, in this typical leaky room, you’d have to crank it up to a much higher air-volume throughput.

A HEPA filter is a beautiful device.  It would work wonderfully in a hermetically-sealed room.  But in an actual room, with high-volume exchange of air between inside and exterior, it just can’t keep up.  You’re better off using a cheap box fan on low (1000  CFM) and a low-back-pressure air electostatic air filter, such as a 3M Filtrete (r) filter.

Is this a fair comparison?  Judging from what I see on Amazon, I’d say so.  When they even bother to show the approximate air flow rate, HEPA units offered as whole-room units typically run at:

Whereas the box-fan-and-filter turns over the air in my example room about 20 times per hour, at roughly 1000 cubic feet per minute.

Further, it makes almost no difference whether I use 99.5% efficiency or 99.9% efficiency for the HEPA unit.  At slow rates of air turnover, the HEPA filter gets overwhelmed by the infiltration of outside air.

Conclusion

I just sent my daughter two Filtrete 1900 filters.  Plus, oxymoronically, a stylish 20″ box fan.  Hoping that on low, the fan will be quiet enough not to be bothersome.

My final finding is that the folks who run Amazon don’t miss a trick.  If you search for a stylish box fan, Amazon suggests a few packs of MERV-13 filters, as an add-on purchase.

My conclusion from the above is that, between viruses and soot, a whole lot of people have figured out that the best and cheapest way to filter indoor air is with some form of “Corsi box”.  So these days, as soon as you pick your fan, Amazon is right there, suggesting the add-ons you need to do that.

Post #1793: Breathing the air is like smoking ___ cigarettes a day.

 

I read an interesting comment in a NY Times article today, the gist of which is that breathing the recent smoky air, at its peak, was like smoking six cigarettes a day.

And I thought to myself a) sounds like somebody made that up, and b) is that even remotely close to being true?

Short answer:  No, not even close. At least, not if you’re just tallying up the total weight of particulates inhaled.  See the last section.

Part of the ambiguity in this question is whether you’re simply talking about the total weight of particulate matter inhaled — a fairly direct calculation — or whether you are trying to infer the net impact on health, from those particles — a far less clear calculation.

Virtually every estimate you will read, comparing air pollution to cigarette smoking, is an indirect estimate that tries to compare them based on presumed effect on health.  Typically, those are based on the observed relationship between cigarette smoking and lifespan, compared to the observed relationship between air pollution and lifespan.  Essentially, those are based on comparisons of epidemiological studies.

Just to be clear, those indirect estimates based on health effects appear to show a vastly higher equivalence between air pollution and cigarette smoking.  That is, they make the claim that routine levels of air pollution are the equivalent of smoking several cigarettes a day.  That’s much higher than you would get, simply calculating the weight of particles inhaled from air pollution, versus from smoking.

I address that at the end of this post.

For now, I just want estimates of the total weight of particles.  What weight would have been breathed in, in 24 hours of New York City’s worst air.  And how much do you inhale, if you smoke a cigarette?

If you do the math, you’ll find that spending 24 hours in the worst of NY City’s recent bad air meant inhaling the same quantity of particulates you’d get from smoking about one cigarette.

By contrast, commonly-used air-pollution-to-cigarette equivalences suggest a vastly different far more eye-popping equivalency.  New York’s peak PM 2.5 reading of 460, if maintained for 24 hours, would be roughly the equivalent of smoking a pack of cigarettes, using the commonly-cited Berkeley Earth estimate that 22 micrograms/cubic meter PM 2.5 is the equivalent of one cigarette per day (reference).

Suffice it to say that, as a health economist, I find the method they used to derive that to be subject to some uncertainty.  Just Google air pollution deaths, and you’ll see that even the most basic estimates are all over the map.

So I’m sticking to something far more basic.  What’s the weight of inhaled particles, compared to what you’d get from a single cigarette.

Details follow.

The calculation

Determining the total weight of air pollution particles inhaled is straightforward.  Take the concentration per cubic meter, and multiply by the average number of cubic meters a person breathes in a day.

(This of course ignores the fact that you exhale some of those particles.  So this isn’t the weight that you retain in your body.  It’s just the amount that you inhaled.)

The clear-enough answer here is that 24 hours of breathing the air, at the peak of New York City’s bad air, would have resulted in inhaling about 11 milligrams of total particulate matter.

That’s an estimate for particles of all sizes.  If  you focused narrowly on PM 2.5, it would be about 5 milligrams.

Now for the hard part.  How much particulate matter do you inhale when you smoke a cigarette?

You can get a hint that my answer is going to differ quite a bit from the “health impacts” answers just by looking up the weight of a cigarette.  For reference, a typical cigarette weighs a gram, or 1000 milligrams, per the WHO. Of that, about 15 percent if water (reference).

So the dry organic matter in just one cigarette weighs about 70 times as much as all the particulates of all sizes that would have been inhaled if you lived one entire day at the peak of New York City’s recent air pollution.

The only question is, what part of that weight ends up being inhaled as cigarette smoke?

Estimate 1:  10 to 40 milligrams (cited in this reference).

In short, you would inhale as much particular matter from smoking one cigarette, as you would living for 24 hours at the peak of New York City’s recent smoky air.

To benchmark, an estimate based solely on inhaled weight appears to equate PM 2.5 level of 85 with about 0.3 cigarettes a day (cited by Berkeley Earth).  That would make New York’s peak of 460 equivalent to about 1.6 cigarettes per day.

So, either by my direct calculation, or by reference to a different calculation, the weight of particulates inhaled at the peak of the recent bad air would be in the neighborhood of the dose you’d get from smoking … roughly one cigarette.

Why the big discrepancy?

I’m pretty sure my calculation of the weights of materials is roughly right.  And I’m still pondering the huge difference between the air-pollution-to-cigarette equivalency based on weight of particulates inhaled, and that based on apparent health effects derived from epidemiological studies.

Again, let me emphasize that the commonly-cited Berkeley Earth estimate (based on apparent impact on health) is an order-of-magnitude higher than the simple estimate based on weight of materials.  That shows up here (where the Berkeley-style estimate would be a pack (20 cigarettes) a day, versus my weight-based estimate of one cigarette a day.  And it shows up in Berkeley Earth’s estimate of the impact of air pollution in Beijing, where their estimate was again about an order-of-magnitude higher than an estimate simply based on weigh of particulates inhaled.

Here’s the weird bottom line.  If we assume that both results are right — both the estimate by weight, and the estimate by apparent impact on health — then this probably implies one of two things.

First, if there’s a linear dose-response relationship — if twice as much smoke is twice as bad for you — then this discrepancy would imply that air pollution particulates are vastly more toxic than cigarette smoke.   Roughly speaking, if both estimates are right, a milligram of particulates from air pollution has the same toxicity as 10 milligrams of cigarette smoke.

That does not seem quite plausible, to me.  Possible, sure.  But my understanding is that cigarette smoke is some pretty toxic stuff.  I find it hard to think that (e.g.) burning coal in a power plant (plus smokestack scrubber), or wood in a forest fire, could be that much materially worse than cigarette smoke, where almost all the particulates are the fine PM 2.5 particulates.

Alternatively, we could explain the same results by a declining dose-response relationship.  Cigarette smoking, in the U.S., consists of a small number of adults who get a large daily dose of smoke.  Air pollution particulates, by contrast, consists of everyone getting a small daily dose of smoke.  If the bulk of the health impacts come from that first little bit of smoke … then sure, you could get small amounts of air pollution equaling the health impacts of large amounts of cigarettes.

And, data from cigarette smokers supports that.  Here’s a study of persons who were non-smokers, occasional (non-daily) smokers, and daily smokers.

The occasional smokers consumed just 8% as much tobacco as the daily smokers.  But with that modest rate of consumption, they incurred almost half as much excess mortality risk.  For smokers, it absolutely is true that most of the health impacts come from just a little bit of smoking.

Stated another way, smoking a pack a day (600 a month) is only about twice as bad for you as smoking about 2 cigarettes a day.

And that, I think, it was explains the big discrepancy between air pollution estimates based on weight of particulates inhaled, and estimates based on presumed health effects of smoking versus air pollution.  As-consumed, it probably is true that the typical milligram of cigarette smoke has much less additional health impact than the additional milligram of PM 2.5 in the air.  But that’s only because once you’ve had your first couple of cigarettes of the day, the damage is done, and the remaining pack or two hardly matters.

(And so, weirdly, if you’d filled in the blank above with 2 cigarettes a day, or you filled it in with 20 cigarettes a day, really, there isn’t much difference between those two answers.  In real, actual, cigarette-smoking terms.  That’s per the table just above.  What you can’t really do, strictly speaking, is to do that equivalency calculation based on simple averages, and expect the results to be meaningful as an expression of actual cigarette smoking.  That’s because the cigarettes-versus-health curve is so strongly non-linear.)

The more I look around on this issue, the more I see that scientists seem to be coming to more-or-less the same conclusion.  The problem with airborne particulates isn’t the occasional peak days, as we just had.  It’s that even low, chronic levels of PM 2.5 in the air can lead to significant negative impact on health, when inhaled over a lifetime.

Post #1792: It is safe to breathe the air yet?

 

In my last post, I noted that yesterday’s readings for airborne particulates in my area were high, but would not have been hugely abnormal in the 1960s.  Yesterday, the air was somewhat visibly smoky, and by mid-afternoon the level of PM 2.5 particles was about 200 (micrograms per cubic meter of air), and PM 10 was about 300 (same units). Continue reading Post #1792: It is safe to breathe the air yet?

Post #1791: Wheezing geezers! It’s a smog alert!

 

I’m old enough to recall when America stood tall, and produced its own air pollution.  Instead of having to import it from Canada.

To cut to the chase:  Air pollution in the DC area is at an extreme level today.  This, owing to Canadian forest fires.  But the level of particulates in the air — currently a PM 10 reading of around 300 (micrograms per cubic meter) — would not have been hugely unusual in the 1960s.  Back then, each year, about 10% of U.S. cities would have seen at least one day with particulates roughly at that level. 

It just takes a bit of work to find the data, and translate the obsolete measure (total suspended particulates) to the modern data (PM 10).

And so, as with our recent “extreme” winter weather, the long-term trend obscures the fact that things were much different in the recent past.  Cold weather that triggers alarms today would have been a yearly occurrence three decades ago (Post #1664).   And particulate levels that result in cancellation of outdoor activities today, would, in the 1960s, have been —  not common, exactly — but frequent enough that all of us of that generation recognize the term “smog alert” as shorthand for a day with bad air pollution.

 

Back when I was a kid …

I had an funny interaction at my bank this morning.  Had to get a document notarized, and ended up chit-chatting with the Notary Public.

Talk turned to the unhealthful air today, the result of smoke from Canadian forest fires.  Said smoke now blanketing much of the U.S. Northeast.

In all innocence, I said something like “this reminds me of my youth.”   My assertion being that, back in the 1960s, in the Washington DC area, we routinely had summertime air that looked about like the air we have today.  Visibility was a mile or two.  Beyond that, everything sort of faded to a gray-white.  The summer sky was always a uniform fish-belly white, from the combination of humidity and particulates in the air.

That opaque air,  in turn, was due to the routinely high levels of air pollution.  That era predates pretty much every form of air pollution control.  Power plants burned coal, and nobody had heard of smokestack scrubbers. Catalytic converters for cars didn’t come in until 1974, so unburnt hydrocarbons ruled the suburban air.  And car fuel systems were open to the air — you simply dumped gas vapors every time you filled up.  Worse, carburetor bowls were vented directly to the atmosphere, leading to continuous dumping of gasoline vapors as you drove or parked.

In this area, the result was massive amounts of photochemical and other types of smog.  Or “haze”.  In the summertime, the most common weather report was hot, humid, and hazy.

But the notary was a young guy, and he frankly did not believe what I was saying.  Having grown up well after the mid-1960s passage of the Clean Air Act, he could not conceive of a world where the current level of air pollution was considered — well, not normal, exactly — but not uncommon, either.

So that’s the task for today.  Was the air in this area routinely hazier back in the day, or is it merely my memories that have become hazy?

Historical Air Pollution Levels

For sure, the U.S. had some dramatic, short-lived air pollution events in that era.  Today, newspapers are recalling the Great New York Smog of 1966.  Such extreme but short-lived smog events were common in New York city, in that era.  Pollution levels during those smogs equaled or exceeded current levels.

And, of course, Los Angeles famously had a unique problem owing to geography.  Air trapped in the Los Angeles basin would more-or-less accumulate everything emitted into it, leading to chronic visible smog.

But I’m looking for information on day-to-day air pollution and visibility levels, going back to the 1960s, ideally in the summertime, in the  Washington DC area.

The problem is that, at the very best, I can find modern-format going back as far as 1980.  That appears to be the year when U.S. EPA  put into place the National Air Quality System (reference), which is an arrangement for gathering and storing air quality data from monitoring stations all around the U.S.

Worse, older EPA air quality data use an outdated measure of particulates.  Currently, we track PM 2.5 and PM 10, the numbers referring to the maximum size of the particles (in microns?)  By contrast, EPA  data from the 1960s uses Total Suspended Particulate, which apparently corresponds to something like PM 50, and is routinely several times higher than the current PM 2.5 or PM 10 measures.  That said, when the annual 90th percentile of maximum daily total suspended particulates was around 400 (micrograms per cubic meter), it’s clear that there was a lot of stuff in the air back then. 

Source:  1973 national air quality report, downloaded from this EPA page on historical air quality reports. Number EPA-450/1-73-001-a.

By contrast, today’s air quality crisis is due to PM 2.5 concentrations around 200 (micrograms per cubic meter), and PM 10 concentrations around 300 (same units).

Source:  Accuweather, data for the DC area, 2 PM 6/8/2023

Luckily, the EPA reports themselves provide a rough crosswalk between the older total particular matter and newer PM 10 measures.  Based on a comparison of the 1990 (left) and 1991 (right) reports, PM 10 appears to run about 2/3rds the value of total suspended particulates, as measured by the EPA.

Source:  1990 and 1991 EPA air quality trends reports, from this page at the EPA.

The upshot.

Today’s PM 10 concentration of 300 (micrograms per cubic meter) is equivalent to a reading of about 450 (same units) for total suspended particulates. Which is just slightly higher than the 90th percentile figure from the 1960s, from the graph above.

In other words, back in the 1960s, every year, 10% of U.S. cities would have seen an air pollution day that was nearly as bad as what DC is experiencing today. 

That doesn’t mean that the average was that bad.  It really means that “smog alerts” were not unheard-of, when I was a kid.  And that, thankfully, that’s no longer true, so the current generation no longer has to treat them as just another fact of life.

I still have not found the data to address the main question of visibility.  Was the air routinely as opaque as it is today, back when I was a kid?

Currently, Dulles Airport is reporting visibility of 2 miles.  But apparently, visibility is only kept in the raw hourly observations, and so far, I have not been able to find 1960s visibility data for my area, from any source.  If I find it, I’ll post it.

 

Post #1788: Recycling plastics, Part 2: My Town tells me to do the wrong thing. Does yours?

 

I am in the middle of looking at plastics recycling in my area.

Any internet search in this area feeds you a lot of pessimism about the entire concept of plastics recycling.  People say that it’s not worth doing, that it’s greenwashing, that it’s a scam, that it all ends up in the landfill, and so on.

But is that true?  It all seems to start from a figure that just 5 to  8 percent of U.S. waste plastic is recycled.

Less than an hour of internet search, and I now know that figure is totally irrelevant to the situation I’m investigating.  The often-cited 5% is for every conceivable form of plastic waste — stuff that was tossed in the trash, stuff that was tossed on the ground, plastic resins that are not recyclable, plastic items that are not inherently recyclable, plastic integrated into multi-material items, and so on.

That’s a problem, for sure.  But right now, I just want to know what happens if I properly handle a recyclable plastic object, where I live.  I want to know two simple things:

  • What plastic should go in the recycling bin, here in Vienna, VA, and
  • What fraction of (say) a clean #1 (PETE) bottle actually gets recycled?

Continue reading Post #1788: Recycling plastics, Part 2: My Town tells me to do the wrong thing. Does yours?