**Background:** The COVID-19 pandemic, caused by SARS-CoV‑2, was first identified in December 2019 in Wuhan, China, and declared a global pandemic on 11 March 2020. As of early February 2023, over 755 million confirmed infections and nearly 7 million deaths had been reported worldwide. While the origin of SARS-CoV‑2 remains under investigation, evidence points to a zoonotic event, likely involving an intermediate animal host. The pandemic highlighted dramatic human encroachment on the environment. This review discusses how environmental factors—including mobility, climate, geography, industrial pollution, urban lifestyle, chemicals, microplastics, and nutrition—influenced the spread and severity of COVID-19.
**Methods:** This is a narrative review that synthesizes findings from multiple published studies on environmental determinants of SARS-CoV‑2 transmission and COVID-19 severity. The authors discuss evidence from epidemiological studies, environmental monitoring data, and observational reports across different geographic regions.
**Key Results:** Modern mobility and transportation systems were major drivers of rapid global viral spread, with dense flight schedules enabling transmission across continents within hours. COVID-19 outbreaks occurred on ships, including a cruise ship in Yokohama and a French military aircraft carrier. Countries above the equatorial line experienced faster spread and higher mortality rates compared to the southern hemisphere. Increased COVID-19 incidence was reported between 0 and 17°C. In the Middle East Gulf States, an increase in relative humidity was associated with a decrease in daily COVID-19 cases and deaths. Patients admitted on days with higher temperatures, higher solar radiation, and lower humidity had higher mortality rates. Long-term exposure to PM2.5, NO2, and O3 was associated with an increased risk of COVID-19 infection. After a sandstorm in Riyadh, Saudi Arabia, elevated concentrations of O3, CO, and PM2.5 were measured, coinciding with a significant increase in SARS-CoV‑2 infections, leading to the hypothesis that air pollutants act as viral vectors. A short-term increase in PM2.5 concentration of 10 μg/m³ increased the number of daily confirmed cases by 9.41%. Poor indoor air quality, inadequate ventilation, and overcrowding in low-income and multigenerational households increased transmission risk. During the pandemic, increased use of disinfectants led to higher indoor concentrations of quaternary ammonium compounds, which impair cholesterol synthesis, increase inflammation, and reduce mitochondrial function. Global plastic consumption increased dramatically due to protective measures and personal protective equipment, leading to increased microplastic pollution. Microplastics can persist in the environment for hundreds of years and, upon accumulation in the human body, may cause physical damage, inflammation, oxidative stress, and DNA damage, potentially increasing susceptibility to diseases like COVID-19. Dietary changes during the pandemic shifted toward ultra-processed carbohydrate-based foods, contributing to obesity, which is a risk factor for severe COVID-19. Daily physical activity was severely restricted, and increased sleep disturbance and lack of social contact further impaired immune response.
**Clinical Implications:** This review emphasizes that environmental factors, particularly air pollution, are significant modifiable risk factors for COVID-19 transmission and severity. There is a striking pathophysiological overlap between SARS-CoV‑2 and air pollution, as both affect the endothelium and immune system. Reducing industrial emissions and improving air quality, especially in densely populated areas, could be critical in reducing respiratory and cardiovascular disease and reducing the risk of severe COVID-19. The findings support stronger environmental health policies to prevent individual diseases and mitigate future social and economic losses from pandemics. Addressing environmental determinants such as air quality, urban planning, nutrition, and chemical exposures should be integrated into public health preparedness strategies.