Epidemiological studies of Covid-19 have, in most cases, provided inconclusive results to date regarding the role of air pollution in the transmission of the SARS-CoV-2 virus and the geographical spread of the disease, both regionally and globally. The present review analysed just under 6 000 articles published up to 31 May 2021, looking at the role of pollution and air quality factors in the transmission of SARS-CoV-2 and in the geographical differences in Covid-19 propagation. A body of evidence shows that chronic and short-term exposure to different fractions of aerosols and types of air pollution exacerbates symptoms, affects co-morbidities and increases mortality rates for respiratory diseases similar to Covid 19, as well as for Covid-19. Although other pathways can contribute, the airborne route is likely to be the dominant mode. There is consistent and increasing evidence that SARS-CoV-2 spreads by airborne transmission, and it is possible that different variants have different environmental sensitivities. Safer indoor environments are required, not only to protect unvaccinated people and those for whom vaccines fail, but also to deter vaccine-resistant variants or novel airborne threats that may appear at any time. The public health community, governments and health agencies should act accordingly, referring to this mode as the principal mode of transmission in their recommendations and statements, enhancing associated research and improving monitoring networks.