The science behind the rating scale
The SARS-COV-2 virus is aerosolised when infected people breath out. The virus hangs in the air in an inside space for hours.
Infected individuals start being infectious up to 4 days before symptom onset and can remain infectious for 8 to 10 days after. Infectiousness peaks at time of symptom onset. 55% of onward transmission happens from people not showing symptoms. Home tests typically only show positive 2 days after a PCR test shows positive and stop showing positive again 1 to 2 days before a PCR test, this is just because home tests are not very sensitive.
Infection is through inhaling breathed out air, the greater the viral load the higher the risk of infection and the greater the risk of that infection leading to long term illness
CO2 monitors are useful to monitor air quality - they are a proxy, they do not measure the quality of the air but they measure what % of the air is breathed out air so are a good gauge of whether there is enough ventilation. Note closed windows and air cleaning will make air safe but will have a high CO2, so not perfect but inexpensive and a good proxy for fresh air. Also a great way to show customers you care about air quality.
Infection is not through surface contact and not limited by proximity. Note face to face time with an infected person carries a high risk and masking is really the only way to make this safe (Far UV aside). Things that affect risk of infection are size of room, number of people in the room, time spent in the room.
The simplest way to reduce the viral load is to circulate the air in a room with fresh outside air.
HEPA filters directly remove the virus from the air and typically achieve the equivalent of 4 to 5 air changes each hour. As noted above they don’t have a huge impact on close face to face contact, so for example for a dentist definitely not enough, but for a general consulting room they will make the space much safer
Staff who are infectious spending all day inside the premises creates an incredibly high risk of infecting anyone entering that space. Far UV disinfects the air very quickly as it kills airborne pathogens on contact. Germicidal UV has been used for over 50 years but operates across the top of a room as skin contact is not safe, Far UV can bathe the entire room in UV light but it is relatively expensive and careful research is needed to get the correct coverage and power. It is worth saying that expensive is an abstract term here - it involves an outlay that is expensive relative to opening windows and running filters, but compared to constant staff absence through sickness it is cheap.
Part of the disinformation campaign with Covid is to create the impression that the science is unclear, that there are experts on both sides of the debate. This is a standard and unfortunately highly effective element of all propaganda. The truth is that SARS-COV-2 is the most studied virus in history, there are already, just four years into the pandemic, literally hundreds of thousands of published studies, links to studies are included throughout the site and on the resources page. On one side of the debate are scientists all over the world collaborating and publishing peer reviewed studies and research, on the other a small number of doctors and scientists employed by an equally small number of massively funded opaque groups with official sounding names (the same groups that fund climate denial and push for deregulation of all markets). The latter group are the people given a platform on mainstream media and quoted by politicians. When you see a heading on a new story that says “Experts are baffled by …” then one of the known complications of Covid infection, this is one of these paid for experts talking to a paid for journalist. For most people it is easier to dismiss this argument as conspiracy theory because Covid is far worse than anyone ever thought and the reality is we have to change how we live.