Air Quality during Covid-19 Lockdown

Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Preventive Medicine and Public Health, Food Sciences, Toxicology, and Legal Medicine, School of Pharmacy, University of Valencia, Avenida Vicente Andres Estellés s/n, Burjassot, 46100 Valencia, Spain; allomo3@alumni.uv.es 2 Department of Health Sciences, University of Florence, Viale GB Morgagni 48, 50134 Florence, Italy 3 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy; lorenzo.cioni@sns.it 4 Hygiene and Public Health Unit, Department of Public Health, AUSL Imola, Viale Giovanni Amendola 2, 40026 Bologna, Italy; m.cancellieri@ausl.imola.bo.it (M.C.); Maria.M.Morales@uv.es (M.M.-S.-V.) 5 Biomedical Research Consortium in Epidemiology and Public Health Network (CIBERESP), Avenida Monforte de Lemos, 3-5. Pabellón 11. Planta 0, 28029 Madrid, Spain * Correspondence: gadon@alumni.uv.es


Introduction
Air pollution is a mixture of particles and gases whose sources and composition vary spatially and temporally. Air pollution in the environment derives both from anthropogenic and natural sources. While natural sources contribute mainly to air pollution in not anthropized regions (e.g., forest fires and dust storms), the contributions from human activities from anthropized regions far exceed the ones from natural sources . The main problem with most ambient air-polluting emissions is that, even if they derive from localized point sources, their effects are not confined to limited areas. In fact, it has been demonstrated that pollutants can travel long distances in a matter of days under favorable meteorological conditions. Therefore, health and air quality in remote areas can be affected by windblown dust, which contains high Air pollution exposure is one of the greatest risks to health worldwide. It is estimated to be responsible for about 4.2 million deaths around the world every year owing to many serious diseases such as heart disease, stroke, acute and chronic respiratory diseases, and lung cancer. The WHO guideline limits are exceeded in several areas around the world, and it is estimated that about 90% of the world's population is exposed to high air pollution levels, especially in low-and middle-income countries. The COVID-19 pandemic has forced governments to implement severe mobility restriction measures to limit the spread of the virus. This represented a unique opportunity to study the impact of mobility on urban air quality. Several studies which have investigated the relations between the quality of the air and such containment measures have shown the significant reduction of the main pollutants in the urban environment so to encourage the adoption of new approaches for the improvement of the quality of air in the cities. The aims of this entry are both a brief analysis and a discussion of the results presented in several papers to understand the relationships between COVID-19 containment measures and air quality in urban areas.
[1] [1] levels of particles, bacteria, and fungal spores, from desert regions like Mongolia, Africa, China, and Central Asia. Since air pollutants cross national borders over time, it seems clear that it is necessary to address this problem by adopting both global and local control policies to reduce emissions.
It is estimated that about 7 million individuals die worldwide each year due to high air pollution exposure.
Ambient air pollution exposure is one of the greatest risks to health worldwide. It is estimated to be responsible for about 4.2 million deaths around the world every year owing to many serious diseases such as heart disease, stroke, acute and chronic respiratory diseases, and lung cancer. From an economic point of view, the total welfare loss in 2013 amounts to more than US$5 trillion in total welfare. According to the WHO, there is already ample evidence that children are more vulnerable to air pollution because they breathe more often, taking in more pollutants, and their nose and mouth are closer to the ground, which is where some pollutants are present in higher concentrations . It is estimated that about 90% of the world's population is exposed to high air pollution levels, especially in low-and middle-income countries.
. WHO guideline values ( Table 1)   generated a significant reduction in private vehicle trips in the city of Rome (−64.6% during the lockdown) . Community Mobility Reports, which were created with aggregated, anonymized sets of data from users who have turned on the Location History setting from their Google Account, provide useful information for the researchers who want to study the relationship between air pollutants levels and lockdown measures. These Community Mobility Reports aim at providing insights into what has changed in response to the policies aimed at contrasting COVID-19. The reports chart movement trends over time by geography across different categories of places such as retail and recreation, groceries and pharmacies, parks, transit stations, workplaces, and residential . Several studies used human mobility data collected from Google (at country-scale) mobility reports to examine the status of improved air quality in world cities due to COVID-19 that led to a temporary reduction in anthropogenic emissions .
For example, the analysis of the changes in air quality during the COVID-19 lockdown in Singapore used human mobility trends from Google , as well as a study that found empirical evidence for a relation between global vehicle transportation declines and the reduction of ambient NO exposure .
This entry aims at obtaining a broad perspective on the impact of lockdown measures during the COVID-  have focused on the study of the effects of Covid-19 lockdown measures on small-medium cities. For example, Donzelli et al. (2020) studied the association between the mobility restrictions during the COVID-19 lockdown and the air pollutants levels recorded in three Italian cities, Florence, Pisa, and Lucca.

Association between Air Quality and Covid-19 Lockdown Measures
For these aims, the authors also made use of meteorological data on rainfall, wind speed, temperature, relative humidity, and solar irradiance for the lockdown period and the same period of the preceding years. In fact, meteorological parameters have an important role in determining air pollution concentrations, so the authors decided to include these parameters in the analysis. Specifically for particulate matter, it is known that PM levels decrease with an increase in precipitation rate, wind speed, and temperature . No evidence of a direct relationship was observed between the lockdown measures implemented by the Italian government and a reduction of PM in such cities, except in areas characterized by heavy vehicular traffic. This result is in line with the above-mentioned study, in which it was observed that the reductions in a European country of PM are much smaller than in more heavily polluted areas of the planet. Moreover, the study of Dantas et al. (2020) reported an increase in the PM concentrations in two air quality monitoring stations of the city of Rio de Janeiro, Brazil.

Nitrogen Dioxide
The main sources of nitrogen dioxide from human activities are constituted by power generation, traffic, and industry. It represents a relevant precursor of both ozone and particulate matter. Respiratory infections, as well as reduced lung function and growth and symptoms of bronchitis and asthma, can increase in people who are exposed to this type of pollutant. In addition, it is well known that nitrogen dioxide exposure is linked with higher morbidity and mortality rates related to cardiovascular and respiratory diseases.

Ozone
Ground-level ozone (O ), also known as tropospheric ozone, is among the most dangerous photochemical pollutants since people exposed to this type of pollutant are more at risk for the development of breathing problems, asthma, reduced lung function, and respiratory diseases. It falls in the class of secondary pollutants since it is not released directly into the atmosphere from the main sources of urban pollution, differently from what happens from primary pollutants. It is formed by the chemical reaction between carbon monoxide (CO), methane, or other volatile organic compounds (VOCs) and nitrogen [17] [ 2 [27] oxides (NO ); this process occurs in the presence of sunlight. In addition to their role as ozone precursors, CO, VOCs, and NO are by themselves dangerous air pollutants. The main sources of NOx and VOCs are traffic, industrial facilities, and chemical solvents. On the other hand, methane derives from waste, fossil fuels, and the agricultural industry. Aside from its health impacts, O is one of the most impacting greenhouse gases and one of the most significant short-lived climate pollutants.

Sulfur Dioxide
The main sources of sulfur dioxide (SO ) are constituted by the burning of fossil fuels (e.g., oil and coal) and the smelting of mineral ores that contain sulfur. People exposed to SO are more at risk of developing damages to the respiratory system and the functionality of the lungs, as well as eye irritation. It is known that some pre-existing diseases, such as chronic bronchitis and asthma, can be aggravated in exposed

Conclusions
Based on the analyzed papers, we can state that there is evidence from various data and studies that there has been a reduction in air pollution all over the world. Although most of the studies reported significant reductions in the levels of some pollutants, we believe that these results should be interpreted with caution due to the lack of some data in the analysis. It is likely that some studies have overestimated the reductions, for example, not considering the long-term trends as potential confounding . In fact, the  [35] comparisons between the pollution levels of the pandemic year with the previous years are intrinsically subject to distortions due to the simple fact that each year is characterized by different conditions, for example, economic conditions, that have not been duly assessed. With regard to meteorological conditions, the majority of studies took into consideration the atmospheric parameters to reduce the bias in the data analyses. We must also point out that different methodologies were used in the different studies, so that it is hard to compare the different results. For example, regarding the period of times which were compared, we noticed that some studies compared the air pollutants concentrations before and during the lockdown, while other compared the previous year (2019) with the first pandemic year (2020) and still others the mean concentrations of the previous five years (2015-2019). Furthermore, there were differences among the containment measures adopted by different governments, so that it is not easy to compare the reductions of the air pollutants under study directly. Finally, various analytic techniques were used by different authors, which used both classical approaches like statistical tests and more advanced methods like machine learning.
However, we can conclude that the implementation of the lockdown measures reduced the levels of PM and PM in most areas under study. Notwithstanding this, we should consider that this difference is more important in most pollutant areas of the planet, while smaller effects were observed in the European countries in which the air quality is generally better. We must remember that some studies did not observe a decrease in PM levels, and others did not detect a significant difference with the periods of normality mobility. Regarding NO , the reductions appear more marked and more homogeneously distributed in the various areas of the globe than those of PM. Most of the studies reported changes in concentrations not smaller than 50%. However, reductions in NO concentration cannot be conclusively calculated without adjusting for long-term trends, which may partially contribute to confounding bias .