Introduction
The emergence of an unusual pneumonia in early 2020 in China led to the introduction of a new type of coronavirus named COVID-19, which was the cause of respiratory disease. Considering the role of air pollution in worsening COVID-19, the deterioration of the physical conditions of patients, the high potential of big cities in the accumulation of air pollutants, and the importance of environmental, economic, and human valuation in crisis management, this study aims to examine the effect of suspended particles of air pollutants on hospitalization and death rates due to COVID-19. In this regard, the number of hospitalizations and deaths due to COVID-19 in Tehran were compared based on district, month, and year of hospitalization and death from February 2020 to May 2022.

Methods
This is a descriptive-correlational study on data of hospitalized, recovered, and death rates due to COVID-19 in 141 hospitals and medical centers in Tehran from February 2020 to May 2022, which were obtained from the Ministry of Health and Medical Education. During this period, there were 402,465 infected cases. Of these, 35,614 died and 366,851 recovered. The data of 36 air pollution monitoring stations in Tehran (22 affiliated to Tehran Air Quality Control Company and 14 affiliated to the Environmental Protection Organization) was also examined. The pollutants in these stations based on the air pollution index (AQI) included PM10, PM2.5, carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and ozone (O3). To test the research hypotheses, linear regression analysis, Pearson correlation test, and one-way analysis of variance were used in SPSS software, version 26. 

Results
The summary of the model showed that the value of the correlation coefficient (R) between the variables of air pollution and death due to COVID-19 was 0.849, which indicates a strong correlation. The value of the adjusted coefficient of determination (0.618) showed that 61.8% of variances in deaths due to COVID-19 were explained by air pollution. In other words, air pollution predicted the death due to COVID-19. 
The effects of O3, CO, NO2, PM10, and PM2.5 and the AQI on the hospitalization rate due to COVID-19 were significant. The pollutants O3 (β=0.765, t=2.049), CO (β=2.371, t=6.155), and PM2.5 (β=3.984, t=3.747) had a positive effect on the hospitalization rate, while the pollutants NO2 (β=-0.664, t=-2.198), PM10 (β=-0.813, t= -3.137), and the AQI (β=-2.806, t=-3.524) had a negative effect on the hospitalization rate.
The effects of O3, CO, NO2, PM10, and PM2.5 and the AQI on the death rate due to COVID-19 were also significant. The pollutants O3 (β= 0.840, t=2.203), CO (β=2.461, t=6.256), and PM2.5 (β=4.196, t=3.865) had a positive effect on the death rate due to COVID-19, which indicates that the increase of these pollutants increases the death rate. The pollutants NO2 (β=-0.736, t=-2.387),  PM10 (β=-1.102, t=-4.166), SO2 (β=-0.312, t=-1.183), and AQI (β=-2.658, t=-3.270) had a negative impact on the death rate, which indicates that the increase of these pollutants causes a decrease in the death rate.

Conclusion
Air pollution affects the hospitalization and death rates due to COVID-19. To control and reduce the amount of air pollutants in metropolitan cities such as Tehran, the changes in air pollutants should be monitored regularly and continuously using satellite data, which has a lower cost and a higher work speed compared to other methods. Health care services and resources should be devoted more to the most infected areas to prevent adverse health effects in other areas. Strengthening environmental policies to reduce air pollution can mitigate the adverse effects of the COVID-19 pandemic and possibly other pandemics that may arise in the future.

Ethical Considerations

Compliance with ethical guidelines
All ethical principles, such as obtaining informed consent from the participants, were considered. Ethical approval was obtained from the Ethics Committee of Islamic Azad University, Yazd Branch (Code: IR.IAU.YAZD.REC.1400.037).

Funding
This article was extracted from the PhD thesis of Alireza Nazarian, Islamic Azad University, Science and Research Branch. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for profit sectors.

Authors' contributions
All authors contributed equally to the conception and design of the study, data collection and analysis, interpretation of the results, and drafting of the manuscript. Each author approved the final version of the manuscript for submission.

Conflicts of interest
 The authors declared no conflict of interest.


 

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