I thought that a lot of businesses, governments and even individuals were spending a lot more time considering the filtering systems of their air circulation systems, whether we are talking about buildings, enclosed recreation, or enclosed transportation systems. 

Of course, some smaller buildings or less trafficked places might not have air circulating systems, but I bet that the consultation and even the production of filters and filterting systems for buildings, enclosed recreation, and enclosed transportation systems has been becoming a booming kind of business, even if it might NOT exactly be a bit clear regarding how much of a difference (based on evidence and logic) that such filtering systems might make to spreadability of viruses or possibly other health benefits as compared with costs of such possible filtering systems.