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Instead of the classic smell of burning wood in a fireplace or campfire, things are starting to smell more like burned plastic so what is going on? It turns out that when smoke stays in the air for prolonged periods of time, the UV radiation from the sun interacts with all the volatile organic compounds (VOCs) to form even more toxic gases. 3/

Now benzene and formaldehyde are being created in the atmosphere from the smoke + UV radiation which happen to smell like plastic but are toxic and can have very adverse effects on human/animal health ( https://www.theweathernetwork.com/en/news/weather/severe/smelling-plastic-millions-are-in-the-path-of-toxic-smoke-plumes ). H/T: @lisa_iannattone

You can learn more about formaldehyde here ( https://www.epa.gov/indoor-air-quality-iaq/what-should-i-know-about-formaldehyde-and-indoor-air-quality ) and benzene here ( https://emergency.cdc.gov/agent/benzene/basics/facts.asp ).4/

Besides the PM2.5 particles in the smoke that can get into your lungs and bloodstream, wildfires smoke it also full of various toxic gases in the form VOCs and by-products like formaldehyde and benzene as we saw above. 5/

Well fitting respirators like N95 masks will do a great job at filtering out the PM2.5 particles from the air if you are outside, but they are not designed to filter out all the smoke related gases. You can learn more about how filtration material in N95s and HEPA filters actually filter out the tiny PM2.5 particles despite them being smaller than the holes in the material ( https://mstdn.science/@jeffgilchrist/109513395114888571 ). 6/

While it is much better to wear a good mask outdoors during wildfire smoke events that filter the PM2.5 particles than nothing, you are still getting exposed to other toxic gases. There are elastomeric respirators with special cartridges that will filter out both the PM2.5 particles and also various gases which would provide better protection outside from smoke ( https://www.cdc.gov/niosh/npptl/respirators/elastomeric/default.html ). 7/

If N95s can effectively filter out tiny smoke PM2.5 particles, why can you still smell things while wearing a good mask? While PM2.5 particles are 20x smaller than the width of a human hair, the gases you smell can be billions of times less atomic mass than PM2.5 particles. 8/

A dalton (Da) or unified atomic mass unit (u) is commonly used in physics and chemistry to express the mass of atomic-scale objects such as atoms, molecules, and elementary particles ( https://en.wikipedia.org/wiki/Dalton_(unit) ).

The mass of formaldehyde is 30 Da while benzene is 78 Da. Compare this size to a single virus particle (virion) at 6 billion Da or an aerosol droplet at 160 billion Da, both monstrous sizes in comparison ( https://twitter.com/jeffgilchrist/status/1602994130086682624 ). 9/

That is why molecules like oxygen (16 Da) can get in through respirators to you and exhaled carbon dioxide (44 Da) can get out so you don't suffocate. But it also means that toxic gases from wildfire smoke can get through N95 masks and why you can smell them so you may still want to limit your time outdoors during smoke events even while wearing a mask. 10/

Another thing I discovered is that Canada's Air Quality Health Index (AQHI) is not the best measure to use for wildfire smoke events. Ontario's AQHI readings especially are problematic since the index actually weighs other pollutants much more than PM2.5 and was designed for urban car/factory pollution which builds slowly so they only look at 3 hour averages while wildfire smoke can jump quickly ( https://www.thestar.com/news/canada/2023/06/29/the-federal-government-has-a-better-way-to-measure-smoky-air-but-ontario-isnt-using-it.html ). 11/

Earlier today the Ontario AQHI level was rated at 4, the first level of moderate risk on a scale that only goes up to 10 ( http://www.airqualityontario.com/aqhi/today.php?stationid=51001 ). 12/

The USA has a PM2.5 specific air quality index (AQI) that goes up to 500. While the Canadian index is showing 4 (yellow), the first level above low risk, the USA index was showing 158 (red) which they rate as unhealthy and the levels of concern increase much more rapidly ( https://www.airnow.gov/aqi/aqi-basics/ ). 13/

For smoke, it is better to look at the actual raw PM2.5 levels to see what is going on so you don't have to worry about conversions. This time last year PM2.5 levels in Ottawa would typically range between 2-6 ug/m^3. Right now in Ottawa raw PM2.5 levels are 70 ug/m^3 which many air quality sites will provide PM2.5 such as PurpleAir here ( https://map.purpleair.com/1/m/i/mPM25/a10/p604800/cC0?select=94235#11.8/45.37764/-75.65956 ) and even the Ontario air quality stations which is better than following their AQHI version ( http://www.airqualityontario.com/history/pollutant.php?stationid=51001&pol_code=124 ). 14/

Since healthcare professionals are warning that there are no known safe levels of PM2.5 it would be prudent to limit exposure as much as possible. You can read more about the equivalent amount of cigarettes at various levels of PM2.5 here ( https://mstdn.science/@jeffgilchrist/110610646117356010 ). 15/

For people in Europe to get a better idea of the size of just one of the largest fires in Quebec right now, it is 12,599 km² (about 88% the size of Northern Ireland 14,130 km² or 57% the size of Irish province of Ulster 22,067 km²). 16/

A tool from NASA ( https://firms.modaps.eosdis.nasa.gov/usfs/map/#t:adv;m:advanced,custom;d:2023-06-03..2023-07-03;l:noaa20-viirs,viirs,modis_a,modis_t,countries;@-73.8,49.1,6z ) can help visualize and measure the size of fires in Canada thanks to @danwalker9999 ( https://twitter.com/danwalker9999/status/1675672657512263680 ). 17/