Dear Colleagues,

Air is required by most species on Earth to survive, including human beings. However, most anthropogenic activities add highly toxic substances to the air and make it difficult to breathe. The major air pollutants include particulate matter (PM), black carbon, ground-level ozone (O₃), nitrogen dioxide (NO₂), nitrous oxide (N₂O), sulfur dioxide (SO₂), and carbon monoxide (CO). Owing to the rapidly increasing population, and the establishment of industries and transport types in mainly the most populous countries in the world, the quality of air is decreasing due to the heavy emissions of the aforementioned air pollutants.

Notably, the status of air pollution has been widely linked with the severity of respiratory/lung diseases, including COVID-19. Hence, in order to prevent the use of public transport, keep people in their homes and out of work, and maintain social distancing, nationwide lockdown measures were adopted. In turn, lockdown reduced air pollution (air pollutants and warming gases) in most cities/countries across the world. Despite these facts, literature describing the status of air pollutants in the world’s top megacities and the major populous countries of the world in pre and post COVID-19 lockdown periods (January/February/March 2020 to month/year with COVID restrictions) is scarce.

Given the above information, this Special Issue aims to compile original research papers, perspectives, and reviews that signify the following:

(i) Introduce the significance, composition and quality of air, and its relation with the COVID-19 pandemic;

(ii) Overview of the major contributors of air pollutants;

(iii) Enlighten the population and the transport loads in the world’s top 10 megacities and the major populous countries of the world;

(iv) WHO guidelines for six air pollutants, namely, CO, Pb, N₂O, SO₂, and the major particulate matters and volatile organic compounds in the world’s top 10 megacities and the major populous countries of the world;

 (v) Appraise recent literature on the status of CO, Pb, N₂O, SO₂, and the major particulate matters and volatile organic compounds in pre and post COVID-19 lockdown periods in the world’s top 10 megacities and the major populous countries of the world;

(vi) Critically discuss the overall scenario regarding the status of the major air pollutants in the world’s top 10 megacities and the major populous countries of the world;

(vii) Discuss the potential consequences of lifting the nationwide lockdown and the eventual normalisation of the temporarily halted industries and transport in the world’s top 10 megacities and the major populous countries of the world;

(viii) Topics/themes suggested by the potential contributors.

The outcomes of the discussion may be useful for further strengthening the ongoing policies for curbing air pollution, and would also provoke future studies on the subject.

Dr. Naser A. Anjum
Dr. Muhammad Arslan
Dr. Mohamed K. Mostafa
Guest Editors

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• global air pollution
• air pollutants
• COVID-19 pandemic
• populous countries
• megacities