The Impact of COVID-19 Lockdowns on Air Quality: A Systematic Review Study

Background: The purpose of this article was to review the published literature and evaluate the association between air quality/air pollution and the lockdown/stay-at-home orders during COVID-19 pandemic. Our goal is to identify the various environmental factors, such as urban and rural air quality, which were affected by the lockdown during the coronavirus disease (COVID-19) pandemic. Methods: We searched


Introduction
The coronavirus disease (COVID-19) is an infectious disease caused by the SARS-CoV-2 virus.The COVID-19 pandemic has infected more than 262 million people and killed more than 5.2 million lives worldwide.The coronavirus pandemic has also caused an enormous economic, public health, and social damages.It is important to note that the risk factors of COVID-19 are still under investigations, but some environmental factors, such as urban and rural air pollution and air quality play a vital role in increasing the population sensitivity to COVID-19 pathogenesis (2).The United States National Oceanic and Atmospheric Administration stated almost 30% decrease in nitrogen oxides (NO2) emissions in the urban northeast during April 2020.In addition, there was a huge reduction in the volatile organic compound concentrations (3).
Research studies have proven that exposure to the fine particulate matter PM2.5 may cause a major harmful health effect.Those effects include cardiovascular, respiratory, diabetes, kidney disease mortality and morbidity (4).Additionally, ecological studies indicate that living in areas with higher levels of ambient fine particulate matter air pollution (PM2.5)increase the chance of having a higher risk of adverse COVID-19 outcomes (5).The COVID-19 restrictions have reduced the emission of the primary air pollutants worldwide in general and in the United States particularly due to the decrease in industrial activities and transportation (6).
The coronavirus disease (COVID-19) pandemic has created so many challenges to the United States government to balance economy and public safety.President Donald Trump declared a national emergency on March 13, 2020, where all municipalities and states issued different degrees of stay-athome and/or lockdowns policies matching the local specific conditions.(7).These local policies have impacted the air quality through decreasing the non-necessary energy consumption and transportation.According to the U.S. Environmental Protection Agency (EPA), the air pollutants and the national emissions contains 59% of carbon monoxide (CO), and 74% of nitrogen oxides (NOx, sum of nitrogen dioxide [NO2] and nitric oxide [NO]) as well as emissions from electric generation and the on-and-off road traffic (7).
In addition, the ambient levels of the two pollutants PM2.5 and PM10 (particulate matter with aerodynamic diameters below 2.5 and 10 μm, respectively) might be the most affected by the lockdown (L.W. A. Chen et al., 2020).It is important to mention that the Ozone (O3) is developed in the atmosphere through the photochemical reaction of the volatile organic compounds (VOCs) and NOx.Also, reducing the VOCs and NOx emissions could either lower or lift the ozone (O3) concentrations depending on the local photochemical regime (7).
Our aim is to estimate the association between air quality/air pollution and the lockdown/ stay-at-home orders during COVID-19 pandemic.We sought to identify changes that were made to the air gases, air pollutants and particular matters during the COVID-19 pandemic lockdown.In addition, our goal is to identify the various environmental factors, such as urban and rural air quality, which were affected by the lockdown during the coronavirus disease (COVID-19) pandemic.Finally, we will evaluate whether there was a uniform improvement in air quality during the COVID-19 lockdown.We will also estimate the association between the COVID-19 fatality rate and air quality by dragging our Furthermore, the changes in mobility were correlated with the relevant air quality parameters, such as NO2, which in turn was highly correlated to O3.The study provides data and analysis to support future planning and response efforts in Sharjah (United Arab Emirates) (8,9).Also, previous studies show the large impact of human activities on the quality of air and present an opportunity for policymakers and decision-makers to design stimulus packages to overcome the economic slow-down, with strategies to accelerate the transition to resilient, low-emission economies and societies more connected to the nature that protect human health and the environment (9,10).

Inclusion and Exclusion Criteria
The researchers reviewed the titles and abstracts of the identified citations and identified eligible articles based on the following criteria.The inclusion criteria included any randomized controlled trial, quasi-experimental study, or pre-post study evaluating the effect of the lockdown on the air quality.The included studies measured health outcomes, processes of care and the effect of the lockdown on the pollution rates in the United States major cities.In addition, we excluded studies published in a language other than English was excluded.Furthermore, data that is not related to COVID-19 were excluded (no other disease).

Study Selection and Data Extraction
The process of selection was performed in two steps.In the first step, we read the titles and abstracts of the citations by the search query to screen the articles based on the inclusion/exclusion criteria mentioned above.In the second step, we read the full text of the citations selected by the first step and based on that we decide if the paper is eligible for inclusion.The search criteria did not limit by publication date; due to recency of the field, the earliest eligible article was published in 2020.In addition, the researchers collected the following information from each article that was eligible: author and year, study objective, methods/tools, geographic location, particulate, and gases measured, source of data, samples collected for analysis and major findings and results.Figure 1.shows, a total of 39 articles were identified through the search conducted in PubMed.We first screened the title and the abstract of those 39 articles for eligibility.A total of 24 articles did not meet the eligibility criteria and were excluded based on the title and the abstract review.The 15 remaining articles were assessed in full text for eligibility and data extraction.After a fulltext review, 3 articles were excluded.Finally, a total of 12 selected articles were confirmed for data extraction.We summarize the basic information of the selected papers.For each paper, we evaluated the main aspects which include but are not limited to the following: 1) descriptions of the study design, 2) sample size, 3) air particles, 4) duration of study, 5) COVID-19 mortality rate, 6) control groups, 7) process and outcome measures, 8) statistical significance, 9) effect of the ambient ozone, 10) ambient air pollutants, 11) urban air pollution, 12) ambient PM2.5 and PM10, 13) disparities in nitrogen dioxide pollution and 14) COVID-19 fatality rate.

Results
Among the 12 studies, 5 articles focused on the effect of the air pollution, fine particulate matter, and air pollutants on COVID-19 pandemic's lockdown, while 1 article targeted the relationship between the weather/air quality and COVID-19 death rate during lockdown.In addition, 5 papers focused on the association between the environmental factors, air pollution and air quality and COVID-19 mortality rate.effect of air quality during the stay-at-home order or the lockdown which was occurred in March 2020.One of the study results shows that the COVID-19 lockdown reduced the disparity in air quality between the census tracts with low and high segments of nonwhite population in some of the rural areas of the United States (for example New York city).On the other hand, the racial gap in air quality remained the same with no notable change in urban areas (3).Additionally, some research findings which have massive environmental policy relevance, indicating that mobility reductions by itself may be insufficient to reduce and decrease the particulate matter PM2.5 uniformly and substantially (4).It is important to note that it has been found that an increase in the average PM2.5 concentration was correlated with a relative increase in the COVID-19 test positivity rate.This explains the increase in the number of COVID cases during the period of the wildfire smoke from August to October 2020 (1).Our results show that counties with higher air pollutants (NO2, O3, CO, PM2.5 and PM10) rates were found to be significantly associated with higher rates of COVID-19 mortality rates.Additionally, counties with higher average daily Particulate Matter (PM2.5)tend to have a significantly higher COVID-19 mortality rate (11).On the other hand, counties with higher average temperatures are significantly associated with much lower mortality rates for COVID-19 (12).During the COVID-19 lockdown the Ozone (O3) concentration decreases which caused a decrease in the air pollution and improve the air quality.This makes the outdoor air much healthier and safer for the human lungs.It has been noticed that the ozone levels are negatively correlated with the COVID-19 death rates (13).The lockdown reduced the concentration of the particulate matter PM2.5 which improves the air quality.Our findings showed that an increase in PM2.5 would cause an increase in the risk of hospitalization caused by COVID-19 (12).It is important to mention that the association of PM2.5 and risk of hospitalization among COVID-19 patients was present in each wave of the pandemic.Also, our study analysis suggested that there was higher risk of hospitalization associated with PM2.5 in Black people compared to White people and in those who living in socioeconomically disadvantaged neighborhoods (5).The COVID-19 lockdown reduced the two air toxicants (i.e., nitrogen dioxide or NO2, and benzidine), which caused an improvement in the air quality.Our results proved that there is a relationship between the COVID-19 lockdown and the air quality.It also confirmed the previously reported environmental factors associated with COVID-19 mortality rate (14).Our systematic review results show that the NO2 (one of the air pollutants) concentrations were positively associated with COVID-19 mortality and fatality rates (15).Additionally, after adjusting for co-pollutants, per interquartile-range (IQR) increase in NO2, the COVID-19 case mortality rate and fatality rate were associated with an increase as well.We should note that we did not notice or observe any significant association between the long term exposure to O3 or PM2.5 and COVID-19 mortality and fatality rates per IQR increase (2,11,16).The reductions of CO and NO2 are statistically significant with the COVID-19 pandemic lockdown.It is also significant at two thirds of the sites and tend to increase with local population density.Additionally, the lockdown has a significant reduction on particulate matter (PM2.5 and PM10), which also has a significant impact on the air quality (L.W. A. Chen et al., 2020).The air pollution levels did not significantly change, compared with historical trends (5).In summary, our

Discussion
Our findings suggested that the urban combustion sources such as traffic, may increase susceptibility to severe COVID-19 outcomes due to the long-term exposure to NO2.This is independent from the long term exposure of O3 and PM2.5.Our results also support directed public health actions to protect the highly polluted regions with prominent levels of NO2.The lockdown lowers the traffic emissions and ambient air pollution which will improve the air quality and reduce the risk of COVID-19 case mortality and fatality (11).It is worth noting that exposure to higher levels of the particulate matter PM2.5 plays a very vital and direct role in increasing the risk of hospitalization among COVID-19 infected patients.Our results show that Black race and people who are living in disadvantaged neighborhoods have a higher risk of hospitalization in the setting of COVID-19 due to the untoward effect of PM2.5 (11).
We have also found that wildfires cause an increase in air pollution due to the elevated PM2.5 (13).This will increase the COVID-19 test positivity rate.Our research findings have indicated that PM2.5 from other sources, such as industry and vehicle traffic increase the susceptibility of COVID-19 (1).
Lockdown reduces the emissions of PM2.5 coming from industries and vehicle traffic and hence reduce the air pollution and improve the air quality (1).There is also a direct relationship between weather changes and the US COVID-19 fatality rates.It only appeared with the ozone levels and the minimum temperature.Our study analysis showed an increase in the minimum daily temperature during the lockdown period which also associated with higher COVID-19 fatality rates.In addition, fewer COVID-19 deaths were associated with higher ozone levels (13).
The methodology used in previous studies can be applied to evaluate the impacts of COVID-19 or similar events on people's mobility, air quality and utility consumption at other geographical locations (17) (10).
Particulate matter concentrations show a quite different pattern from the rest of pollutants examined and with substantial week-to-week variations (10)( 17) (8).The current study concludes that due to declining automobile and industrial emissions in The Northern Emirates of the United Arab Emirates (NEUAE), the lockdown initiatives lowered NO2, AOD, and Surface Urban Heat Island Intensity (SUHII).In addition, the aerosols did not alter significantly since they are often linked to the natural occurrence of dust (10).
Our study has various limitations that are worth noting.In some of the eligible articles used in this literature review, the health care, demographic, utilization, and viral testing data were taken from a single institution and collected as part of routine care (18)(19)(20)(21)(22)(23)(24)(25).
Additionally, the electronic health record (EHR) data is subject to bias and error.These errors are hard to be controlled and do not allow us to observe significant changes.Lastly, future studies are necessary to refine our findings and improve our understanding of the effects of the COVID-19 pandemic lockdown on the air quality (6).

Conclusion
The findings indicate that the COVID-19 lockdown has significant impact on the air quality across the world.The lockdown significantly reduces the air pollutants such as NO2, CO, O3, and Particulate Matter PM2.5 and PM10.This reduction led to a much healthier and safer outdoor air and Dahu BM, Aburayya A, Shameem B, Shwedeh F, Maryam A, Aljasmi Sh, Salloum SA, Aburayya H, Aburayya I.The Impact of COVID-19 Lockdowns on Air Quality: A Systematic Review Study (Review Article).SEEJPH 2022, posted: 09 October 2022.DOI: 10.11576/seejph-5929 P a g e 4 | 22 attention on the direct relationship between the air quality and the COVID-19 mortality across the world.

Finally, 1
research study paper aimed to study the COVID-19 positivity rate and the Dahu BM, Aburayya A, Shameem B, Shwedeh F, Maryam A, Aljasmi Sh, Salloum SA, Aburayya H, Aburayya I.The Impact of COVID-19 Lockdowns on Air Quality: A Systematic Review Study (Review Article).SEEJPH 2022, posted: 09 October 2022.DOI: 10.11576/seejph-5929 P a g e 6 | 22 Dahu BM, Aburayya A, Shameem B, Shwedeh F, Maryam A, Aljasmi Sh, Salloum SA, Aburayya H, Aburayya I.The Impact of COVID-19 Lockdowns on Air Quality: A Systematic Review Study (Review Article).SEEJPH 2022, posted: 09 October 2022.DOI: 10.11576/seejph-5929 P a g e 7 | 22 results showed that the COVID-19 locked down which occurred in March 2020 has affective the air quality positively.It has improved the air quality by decreasing the pollution rate.It also helps in decreasing the polluted gases such as NO2, CO, O3, PM2.5 and PM10.Additionally, the air quality affected the COVID-19 testing and positivityrates.This systematic review study has shown that the COVID-19 testing, and positivity rates are higher in the areas with high pollution rates.The lockdown has improved the air quality, decreased the pollution rates, and decreased the COVID-19 mortality and fatality rates.
Dahu BM, Aburayya A, Shameem B, Shwedeh F, Maryam A, Aljasmi Sh, Salloum SA, Aburayya H, Aburayya I.The Impact of COVID-19 Lockdowns on Air Quality: A Systematic Review Study (Review Article).SEEJPH 2022, posted: 09 October 2022.DOI: 10.11576/seejph-5929 P a g e 9 | 22 hence improved the air quality during the lockdown/stay-at-home orders.More research is needed to validate that the air pollutants (NO2, CO, O3, PM2.5 and PM10) have a significant impact on the COVID-19 mortality and fatality rates.The lockdown during the COVID-19 pandemic improved the air quality as well as decreasing the exposure and the emissions of the particulate matter PM2.5.This will help decrease the risk of hospitalization among COVID-19 infected individuals.Lastly, this study indicates the need for public health efforts during the hard hit of COVID-19.It also improves the advantage of the lockdown on the air pollution and air quality.