aerosol transmission

minipost

 

The American Physical Society had a webinar on aerosol covid transmission.   Jose Jimenez of the University of Colorado at Boulder is an aerosol physics expert.    It’s not terribly technical, but the first 45 minutes is probably mostly for public health and medical types. Much  is making the case for about 80% of covid transmission being aerosol rather than droplet.  This was an issue until a few months ago as the WHO and CDC believed it was mostly droplet.  It turns out that was based on a flawed study that is almost 50 years old!  They’ve since recanted.

 

 

Starting around 45m he goes into recommendations.   As always the best bet is to avoid them if people you don’t live with are present or have recently been inside.  With aerosols stagnant air is a problem so you want good ventilation if others are involved.  It turns out CO₂ level in a room is a good enough proxy for aerosol concentration.  Sit in a car with three other people, windows up, and after a short amount of time you’re breathing each other’s air and the CO₂ levels are sky high. Open a window and turn on the ventilation and the levels come down.  He wants to see CO₂ meters in all enclosed public spaces so you can decide what might be safe or not or how long you can be inside.  He’s not a virologist, but some are recommending 700 ppm as a guideline (I've seen 600 mentioned).

 

 

Good enough detectors based on NDIR are in the $100 to $200 range.  It's a very simple technology that uses the fact that carbon dioxide strongly absorbs a specific frequency of  infrared light.  They’re only good +/-50 ppm, but that’s good enough.   Of course all of this has to be balanced with the infection levels in your community.  

 

postscript

 

Several readers asked for CO₂ meter recommendations. I can make a few recommendations, but have only used a few.  Stay away from sub $100 units is the only rule of thumb I can offer.  Schools should have them in each room as well as stores, offices and so on.   You probably don't need one in your home unless you’re curious or want to see how well your windows and HVAC system are working (that can be a big issue).

 

The seminar talks about airplanes. Air exchanges per hour are very high when the ventilation system is on so airborne infection risk is low inside the cabin.  The rest of the trip is questionable - very high levels were found in jetways for example. New York subway trains have good air exchange rates, but I haven’t seen any work on the platforms. Buses vary and I’d be suspicious when the heaters are on in the Winter. I’ve measured a variety of cars. With windows up and two occupants CO₂ levels were over 3500 ppm in twenty minutes. That’s extremely dangerous if one of the occupants is contagious. Opening windows helps, but cars vary. The effect isn’t big on very streamlined cars like Teslas. On aerodynamically dirtier cars opening rear windows canincrease air exchange more than front windows as the outside air pressure on a moving car increases towards the rear of the vehicle. You should also turn on air circulation. With just circulation I got readings around 900 ppm and the best readings with windows were in the 700s.  I’d stay away from any car if someone outside of your bubble is in it.

 

An interesting note is some measurements have been done on cognitive ability and alertness. Over 1500 ppm and people become less alert.  That can have an impact on the quality of work as well as high attention activities like driving. The few studies that have looked at cognitive ability show declines with increasing CO₂ levels.  One study saw the decline start around 800 ppm and another around 1500 ppm.  Neither study was gold standard robust, but I suspect the effect is real - certainly alertness is real.  Many school rooms, offices and homes are normally over those levels so your may want to make changes even after the pandemic.