Here is THE paper to read: Practical Indicators for Risk of Airborne Transmission in Shared Indoor Environments and Their Application to COVID-19 Outbreaks.

Authors: Z. Peng, A.L. Pineda Rojas, E. Kropff, W. Bahnfleth, G. Buonanno, S.J. Dancer, J. Kurnitski, Y. Li, M.G.L.C. Loomans, L.C. Marr, L. Morawska, W. Nazaroff, C. Noakes, X. Querol, C. Sekhar, R. Tellier, T. Greenhalgh, L. Bourouiba, A. Boerstra, J.W. Tang, S.L. Miller, and J.L. Jimenez*

This blog is a very short summary with excerpts from our paper reproduced below. This is a peer-reviewed publication that describes the theory and application of the COVID-19_Aerosol_Transmission_Estimator (spreadsheet here). This El País article is an excellent visualization of this work.

Some infectious diseases, including COVID-19, are transmitted through the air. This may happen at close proximity, but as time indoors increases, infections can occur in shared room air despite distancing. In this paper we estimate risk for COVID outbreaks as well as measles, influenza, and TB.

We propose two indicators of infection risk for this situation, that is, relative risk parameter (H_r) and risk parameter (H). An important advantage of the simple risk parameters is that their values can be calculated for outbreaks that are documented in the scientific literature. 

Let’s just focus on H for now.  It is the number of susceptibles multiplied by the relative risk of infection.  It is an approximate indicator of the absolute probability of infection. It has a unit of person h² m⁻³, which indicates the increase of risk with number of people, with duration (h), the inverse of the ventilation rate expressed as air changes per hour (1/h⁻¹ = h), and the inverse of the volume of the space (1/m³). 

H is an indicator of risk in terms of OUTBREAK SIZE. Low risk: H<0.05; Med: H<0.5; High: H>0.5. The lowest H for the major COVID-19 outbreaks in indoor settings reported in the literature is ∼0.1 person h² m⁻³. H can be orders of magnitude higher for the superspreading events where most attendees were infected (e.g., the Skagit Valley choir rehearsal).

This figure is reproduced from the paper (Figure 2 in the paper) and shows some social settings and the predicted value of the risk parameter H. Least risky is outdoors and minimal vocalizing, while most risky is poorly ventilated and heavy exercising (think spin class at the gym).

All of the outbreaks investigated in this paper concern the early to mid-2020 variants of SARS-CoV-2, and a variant twice as contagious as those should reduce the tolerable values of the parameters by about a factor of 2. 

Substandard ventilation, coupled with poor air distribution, is associated with substantial increases in the risk of outbreak. Mitigation measures to limit shared-room airborne transmission are needed in most indoor spaces whenever COVID-19 is spreading in a community. Among effective measures are reducing vocalization, avoiding intense physical activities, shortening the duration of occupancy, reducing the number of occupants, wearing high-quality well-fitting masks, increasing ventilation, improving ventilation effectiveness, and applying additional virus removal measures (such as HEPA filtration and UVGI disinfection). 

We have shown that combinations of some or all of these measures are able to lower H close to 0.01 person h² m⁻³, so that the expected number of secondary cases is substantially lower than 1 even in the presence of an infectious person, hence would be likely to avoid major outbreaks.