New COVID-19 variants, either of higher viral load such as delta or higher contagiousness like omicron, can lead to higher airborne transmission than historical strains. This paper highlights their implications for health policies, based on a clear analytical understanding and modeling of the airborne contamination paths, of the dose following exposure, and the importance of the counting unit for pathogens, itself linked to the dose-response law. Using the counting unit of Wells, i.e. the quantum of contagium, we develop the conservation equation of quanta which allows deriving the value of the quantum concentration at steady state for a well-mixed room. The link with the monitoring concentration of carbon dioxide is made and used for a risk analysis of a variety of situations for which we collected CO ₂ time-series observations. The main conclusions of these observations are that 1) the present norms of ventilation, are both insufficient and not respected, especially in a variety of public premises, leading to high risk of contamination and that 2) air can often be considered well-mixed. Finally, we insist that public health policy in the field of airborne transmission should be based on a multi parameter analysis such as the time of exposure, the quantum production rate, mask wearing and the infector proportion in the population in order to evaluate the risk, considering the whole complexity of dose evaluation. Recognizing airborne transmission requires thinking in terms of time of exposure rather than in terms of proximal distance. Highlights: Relative airborne risk assessment following variant viral load and contagiousness Indoor analytical risk assessment including absence of ventilation Adequacy of the present norms of ventilation to Covid-19 pandemic Observation of non-compliance to standards concerning CO ₂ Indoor Air Quality

In this paper we develop a simple model of the inhaled flow rate of aerosol particles of respiratory origin i.e . that have been exhaled by other humans. A connection is made between the exposure dose and the probability of developing an airborne disease. This allows a simple assessment of the outdoor versus indoor risk of contamination to be made in a variety of meteorological situations. It is shown quantitatively that for most cases, the outdoor risk is orders of magnitude less than the indoor risk and that it can become comparable only for extremely specific meteorological and geographical situations. It sheds a light on various observations of Covid-19 spreading in mountain valleys with temperature inversions while at the same time other areas are much less impacted. Highlights: Risk of covid-19 airborne transmission. Quantitative assessment of outdoor versus indoor airborne risk of transmission. Meteorological and geographical influence on covid-19 airborne transmission.

Following the outdoor model of risk assessment developed in one of our previous studies, we demonstrate in the present work that long-range transport of infectious aerosols could initiate patient “zero” creation at distances downwind beyond one hundred kilometers. The very low probability of this outdoor transmission can be compensated by high numbers and densities of infected and susceptible people such as it occurs in large cities, respectively in the source and the target.