Mr. Simon Jones:

It is important to remember that there are no absolutes; nothing is 100% perfect. Even if a device is capable of removing 99.9% of viruses in a single air pass, its ability to be effective in a space depends on whether it is appropriately sized. A recent study showed that four or five HEPA filters are needed in a single classroom of a particular size to deliver effective control. The first port of call should always be fresh air, or fresh outside air. Another point to remember is that if air is being recirculated, one may be removing particulates and viruses but one is not dealing with other pollutants. While dealing with one problem, another can build up, such as humidity, VOCs, chemicals in the air, and other pollutants or gases. That is why I say there is a hierarchy. We try to deal with ventilation with a view to removing pollutants in the first place, replacing them with fresh outside air. If that is not enough, we start to look to other technologies. The challenge is determining when fresh outside air is not enough and there is a need to supplement it. If organisations such as schools are to invest in HEPA filters, how do they know they are at a point at which they need to do so? The devices are not cheap for a school. If a school has 20 or 30 classrooms and two or three HEPA filters are needed per classroom, a big investment will be required. How does the school know it is in a position to need those types of products?