Smaller doesn't mean harder to filter, beyond a certain size. HN favorite (and pre-covid particulate filtrering proponent) dynomight explained this well in an article on a cheap air purifier:

https://dynomight.net/ikea-purifier/

Also, I'm pretty sure respiratory viruses need aerosol droplets to get anywhere they can harm you, and those droplets are much bigger than viruses anyway.

Some respiratory viruses need large aerosol droplets, but covid virus particles remaining viable with a half-life of up to about an hour outside large droplets.

The initial theory was that environments like extreme hot and humid climates made it impossible for aerosol droplets to harm people, while dry air increased the risk. This recommendation was change later into the pandemic as new evidence was gathered.

There are similar studies on influenza. There doesn't seem a strong consensus on what environments are safe, or how long such viruses can last outside of large droplets. There is a lot of factors in play, including the virus itself.