Just wanted to share this 2020 (relatively new) review I read today. It's on how outdoor pollen concentration correlates with indoor pollen and with allergy/athma symptoms. It sites numerous studies on how to provide patients with relief, other than with pharmaceuticals. Plenty of references, written in relatively non-technical language. Message me if you can't see the PDF.
karger.com/iaa/article-pdf/...
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Below are some of my observations (there is much more information in the paper). I am not a medical doctor, I am just a desperate allergic asthma suferrer and happen to be scientist by trade.
Turns out, even after the pollen season 'officially' ends, the pollen persists in the settled indoor dust for the whole year -- if one uses window opening to vent (which 99% of people do, except high-end hospitals and ICUs). This is even with weekly wiping of floors and surfaces. The antigen in the dust-derived pollen (the 'active ingredient' in the pollen) also remains active for the whole year. This explains why pollen allergy sufferers may not be feel a full relief during the off-season and start suspecting other sources (mold etc) -- "pollen must be over by now!"
Fabric-like surfaces (carpets, curtains, cotton clothing, uphostery) trap pollen much better than polyester and than surfaces that can be wiped (wood floors and linoleum). Cotton traps pollen most efficiently, even with washing. I found this out by myself by doing particle-counter measurements with different materials, and found that cotton was indeed the worst in terms of dust it produced. All of my bedding is now fleece and polyester, washed at 60C weekly. No carpets anywhere, obviously. Leather is quite good (or so they say), but my wife is vegan, that's a no go.
Hair-washing before going to bed -- I do this anyway.
Wearing masks when outdoors -- I do this anyway.
Not drying clothing outdoors -- duh!
Ventilation: I've been venting by opening windows early in the morning for 10-15 min (until CO2 drops below 600 ppm). However, venting in the morning, is not always a solution, b.c., according to research they cite, pollen counts were found to be high for some trees throughout the 24-hr period. I am now moving towards venting only through a HEPA/carbon filter, by forced ventilation, 24-7 (to pressurize the flat). I have mostly stopped venting by opening the windows during the pollen season (which lasts for 6 months for me). A split AC or a 2-hose AC is a must during hot humid summe days. Single-hose ones are no good: they suck in the outdoor air.
Indoor HEPA filters: the results were inconclusive, but in my case, I believe, filters work if set up right. In my experience, most people just place the filter in the room and consider the job done. I do not think this alone can help much, unless all other measures are taken (removal of allergen-trapping particles, removal of pets, no open windows, pressurized air, forced ventilation through another HEPA etc. Only semiconductor cleanrooms and the most desperate people like myself are ready to implement all of these measures to try and to control the particles in the air.