COVID-19 and its association with meteorological, climate, and environmental factors: A systematic review

Abstract

COVID-19 transmission can be influenced by various factors, including weather and climate conditions, population density, and the availability of medical facilities. To gain a deeper understanding of this topic, an in-depth analysis of recent studies is needed. Our objective was to investigate previous systematic reviews that have examined the seasonal variation of COVID-19 and the impact of climate on its transmission and mortality. Online databases that included PubMed, Scopus, Web of Science, and Cochrane were searched using relevant keywords up to November 2021. Negative associations were found between temperature and COVID-19 spread and mortality (6/9 studies, 66.6%). These negative correlations imply a decrease in the spread and mortality of COVID-19 with an increase in temperature. Similarly, seven systematic reviews reported a negative correlation between humidity and transmission or mortality of COVID-19 (7/9 studies, 77.7%). COVID-19 spread was not associated with precipitation (three studies) but was negatively correlated with sunlight or UV radiation (two studies), COVID-19 incidence and mortality were positively associated with wind speed (one study). One study reported that the effect of air pressure and UV radiation on COVID-19 activity was unknown. The effects of air pollution, seasonal changes, wind speed, precipitation, and UV radiation on COVID-19 incidence or mortality remain unclear. However, factors proposed as having the greatest influence on COVID-19 incidence or mortality include air pollution, wind speed, and wastewater. Sunlight exposure and warm climates likely assist with reducing COVID-19 incidence or mortality, with the infection having a winter seasonality.

Introduction

Coronavirus Disease 2019 (COVID-19), caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), emerged as a global health crisis in late 2019. ¹ Since its declaration as a pandemic by the World Health Organization (WHO) in March 2020, COVID-19 has caused significant morbidity and mortality worldwide. ² While its transmission primarily occurs through respiratory droplets and aerosols, several environmental factors, including meteorological and climate conditions, have been investigated for their potential role in disease spread, and severity. ³

COVID-19 presents with a wide range of clinical manifestations, from asymptomatic infections to severe respiratory failure.^4,5 Common symptoms include fever, cough, dyspnea, fatigue, and anosmia, while severe cases may lead to acute respiratory distress syndrome (ARDS) and multi-organ dysfunction.^{5 –7} The disease disproportionately affects older adults and individuals with comorbidities such as cardiovascular disease, diabetes, and chronic respiratory conditions. ⁸ Socioeconomic factors, including healthcare access and occupational exposure, also influence infection risk, and outcomes. ⁹

Meteorological variables such as temperature, humidity, and precipitation have been studied for their impact on COVID-19 transmission.^{10 –12} Evidence suggests that lower temperatures and reduced humidity levels may prolong viral survival on surfaces and in aerosols, potentially increasing transmission rates. ¹³ Some studies have also indicated that UV radiation could have a mitigating effect on viral persistence, although behavioral factors, such as increased indoor gatherings in extreme weather, may counteract these effects.^13,14

The interaction between climate change and infectious diseases, including COVID-19, has gained global attention. ¹⁵ Climate variability influences human behavior, mobility, and public health interventions. Lockdowns and social distancing measures may be more challenging to implement in regions facing extreme weather conditions, affecting disease control efforts.^16,17 Additionally, environmental degradation and urbanization increase the risk of zoonotic spillovers, contributing to the emergence of novel pathogens. ¹⁸ While numerous studies have examined the relationship between meteorological and environmental factors and COVID-19, findings remain inconsistent due to variations in methodologies, data quality, and confounding factors. ¹⁹ Mathematical models incorporating climate data, pollution levels, and human behavior have been used to predict COVID-19 trends, though their accuracy depends on multiple dynamic factors, including vaccination rates and viral mutations.^20,21

Another possibility for the inconsistent results concerning COVID-19 and environmental conditions is co-infection with other respiratory diseases, such as influenza, which is a group of viruses that was once associated with pandemics. ²² This possibility increases particularly in winter conditions, as host susceptibility and the virus viability increase. ²³ Public health policies must consider environmental health alongside traditional infectious disease control measures to enhance pandemic preparedness. A comprehensive, interdisciplinary approach is essential for mitigating the impact of COVID-19 and future pandemics influenced by environmental and climatic conditions. Thus, this paper aimed to conduct a systematic review to allow for an in-depth analysis of recent studies investigating COVID-19 and its association with meteorological, climate, and environmental factors.

Materials and Methods

In this investigation, we comprehensively reviewed the seasonal variation of COVID-19 and also the effects of climate on its transmission and mortality. These objectives are cultivated by going through available systematic review studies on this topic. To ensure that the results of this paper are accurate and reliable, we adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist.

Data sources

An extensive search of four online databases was performed, which included PubMed, Scopus, Web of Science, and Cochrane. Articles were restricted to the English language, and the search was conducted up to November 19, 2021. To advance the search strategy, we designated various keyword compounds in the formats shown below:

• A. “Novel coronavirus” OR “2019-nCoV” OR “SARS-CoV-2” OR “COVID-19” OR “SARS-CoV2” [Title/ Abstract]

• B. “Weather” OR “Season” OR “Seasonal variation” OR “Ambient” OR “Temperature” OR “Climate” [Title/ Abstract]

• C. [A] AND [B]

Study selection

In order to improve the study selection process, a two-step method was employed. First, titles and abstracts were screened by three researchers. In the second step, the full text of articles identified as being potentially eligible was reviewed by three researchers. The inclusion and exclusion criteria for this systematic review are provided below.

The inclusion criteria:

• − Systematic review publications with or without meta-analyses

• − Publications in the English language

• − Peer-reviewed publications

The exclusion criteria:

• − Original articles, non-systematic reviews, letters to editors, commentaries, protocols, and guidelines

• − Publications lacking full texts, abstract papers, and conference abstracts

• − Publications containing non-human studies

Data extraction

A standardized template was used for data extraction, with this phase involving three researchers. Data extracted included first author (reference) information, study type (systematic review or meta-analysis), country and year of study, date of conducted search, searched databases, number of articles included, effects of conditions (i.e., temperature, humidity, air pollution, seasonal variations, and other reported parameters), and summary of findings. To ensure the accuracy of the extracted data, two other investigators of our team checked the data.

Quality assessment

The quality and authenticity of the included articles were endorsed by two independent research members. In any case of disagreement, a third senior researcher was used to make a final decision.

Results

By the initial literature search, we retrieved 827. From these resources, 12 duplicates were removed and 15 articles were excluded from the initial screened. 791 studies were excluded due to a variety of reasons as follows: non-human studies (n = 23), original articles (n = 63), non-systematic review (n = 44), Abstract/conference abstract/no available full-texts (n = 48), protocol and guidelines (n = 22), and 591 unrelated articles. Finally, nine studies met the inclusion and exclusion criteria (Figure 1). All included studies were systematic reviews and were conducted in a diverse range of countries. The majority of the systematic reviews focused on the effect of meteorology and the environment on the COVID-19 pandemic.

Figure 1.

PRISMA 2020 flow diagram of study retrieval process.

The effects of temperature

Eight of the nine studies investigated the effects of temperature on the incidence, transmission, and mortality of COVID-19 (ID: 1–8). Two studies did not provide a specific conclusion or clear evidence between COVID-19 and temperature (ID: 5, 8). Six other studies have reported a negative association between temperature and the spread of COVID-19.

Zazouli et al., suggested that an increase in temperature would increase the number of people interacting in outside environments, and it may increase COVID-19 cases, ²⁴ Romero Starke et al. found a relationship between increasing temperature and decreased mortality rates of COVID-19. ²⁵ According to Guo et al., the virus inactivation occurs at 25°C–30°C, and its infectivity increases at a low temperature of 5°C and decreases at higher temperatures. ²⁶

The effects of humidity

Eight out of the nine studies investigated the effect of humidity on COVID-19 (ID: 1–8). Zheng et al., reported that some of their included studies found no significant correlations between relative humidity and COVID-19 transmission, and a finding of low positive correlations between specific humidity and COVID-19 spread. ²⁷ However, the other seven studies found a negative correlation between humidity and transmission or mortality of COVID-19. One of the studies identified a specific range of relative humidity between 50% and 70%, which can reduce the SARS-CoV-2 virus in the air ²⁶ ; however, another study mentioned a range of humidity for COVID-19 transmission. ²⁴

The effects of air pollution

Only two of the nine studies focused on the correlation between air pollution and transmission of COVID-19. Zazouli et al., suggested that air pollution is a potential carrier for transmission, which may worsen the COVID-19 effect on health. ²⁴ Short-term and long-term exposures to particulate matter (PM) 2.5 were positively associated with the incidence and mortality of COVID-19, but the evidence was uncertain for short-term courses. Although the effects of exposure to PM10 or O₃ were uncertain in the short term and long term, long-term exposure to NO₂ was certainly associated with higher COVID-19 mortality. ²⁸

The effects of seasonal variations

Other parameters were investigated in seven out of the nine studies (ID: 1–5, 7, 8). The potential impacts of wind speed (five studies), air pressure (one study), precipitation (three studies), sunlight and radiation (four studies), and sewage and waste (two studies) were also reported. Four studies did not find any clear or specific relationship between wind speed and COVID-19 transmission. Rahimi et al., found a positive association between COVID-19 cases and wind speed and an increase in the mortality rate with increasing wind speed. ²⁹ Zheng et al., reported that the effect of pressure and ultraviolet (UV) radiation on COVID-19 activity is unknown. ²⁷ Three studies did not find any significant association between precipitation and COVID-19 spread (ID: 1, 4, 8). Among four studies that investigated the relationship between sunlight or radiation and COVID-19 transmission and spread, two studies reported an unknown relationship (ID: 1, 8), and two studies found a negative correlation (ID: 4, 5). Rahimi et al., reported that the infection risk of polluted water by COVID-19-infected wastewater needs to be investigated. ²⁹ Further, Zazouli et al., showed that sewage and waste containing SARS-CoV-2 can be a potential route for outbreaks. ²⁴ Guo et al., suggested that COVID-19 had strong winter seasonality, ²⁶ which was in line with Saputra et al., results that hot summer and rainy seasons in Bangladesh can decrease COVID-19 transmission. ³⁰ The thorough details of all nine included studies are provided in Table 1.

Table 1.

The effects of temperature, humidity, air pollution, and seasons on COVID-19 incidence, transmissibility, and mortality.

ID	First author	Country & year	Date of the conducted search	Searched databases	The number of articles included	Temperature effects	Humidity effects	Air pollution effects	Effects of seasonal variations and other factors	Summary of findings
1	Zheng et al. 27	Global analysis, 2021	Dec 2019–Feb 2021	Web of Science, PubMed, and Chinese National Knowledge Infrastructure.	62	Higher temperatures may reduce the spread of coronavirus and suppress the pandemic.	No significant correlations were found between relative humidity and disease. (14/37 studies). Low positive correlation between specific humidity and confirmed cases of COVID-19. (3/9 studies) ---- nO specific conclusion.	__	The effect of wind speed and direction cannot provide a specific conclusion. The relationship between pressure, sunlight, precipitation, and COVID-19 activity is unknown. The effect of UV radiation on COVID-19 was unknown. Two studies showed a positive relationship between confirmed cases and water vapor.	Higher temperatures can reduce the progress of the COVID-19 epidemic. However, these climate variables alone could not account for most of the variability, and focusing more on health policies is necessary too.
2	Zazouli et al. 24	England, USA, Italy, Canada, France, Europe, Pakistan, Bangladesh, India, China, Spain, Africa, Australia, Netherlands 2021	Oct 2019–Jul 2021	PubMed, Scopus, Web of Science, Google Scholar, Magiran, and Scientific Information Database (SID)	30	An increase in temperature can reduce the incidence of COVID-19. However, an increase in temperature can increase the presence of people outside the home, which can be a factor in not reducing the incidence in hot seasons.	There is a range of moisture, which is especially suitable for transmitting COVID-19.	It is hypothesized that air pollution can act as a carrier and worsen the effects of COVID-19 on health.	Sewage and waste containing the COVID-19 virus can cause pollution of the environment and water resources, and thus spread the virus in case of a lack of proper management.	Each of the factors of air pollution, temperature, humidity, Water, sewage, and waste can affect the rate of morbidity, infection, and death caused by the Covid-91 disease.
3	Romero Starke et al. 25	Worldwide, China, Pakistan, Bangladesh, and England, 2021	July 2020–January 2021	PubMed, Embase, and Cochrane COVID-19 databases.	11	Potential association between temperature and COVID-19 mortality results is unclear, yet most studies that found a consistent association reported a tendency for decreased mortality with increasing temperature.	Presented a possible negative correlation between mortality and humidity.	__	Wind speed data cannot provide a specific conclusion.	Despite the inconsistent results, most studies imply that a decrease in temperature and humidity contributes to increased mortality.
4	Saputra et al. 30	India, Turkey, Japan, Bangladesh, Indonesia, Iran, China, and Gulf Countries, 2021	2020	ProQuest, Scopus, PubMed, and SpringerLink databases.	11	High temperature (p = 0.038) significantly reduces the transmission of COVID-19, and the temperature was significantly correlated with the incidence of daily new cases with and without time lag.	High humidity significantly reduces the transmission of COVID-19	__	Precipitation: Cannot provide a specific conclusion. Wind speed: Cannot provide a specific conclusion. Sunlight exposure: A strong negative correlation between solar radiation and the incidence of COVID-19. Long duration of sunlight exposure was also associated with a higher case recovery from COVID-19 in patients. The arrival of the rainy season and hot summer in Bangladesh can reduce the transmission of COVID-19.	A positive association between temperature, wind speed, humidity, average precipitation, and the number of sunny days with COVID-19 shows that climate plays a role in the spread of the COVID-19 pandemic in Asia.
5	Rahimi et al. 29	Iran, USA, China, England, Indonesia, India, Netherlands, Australia, 2021	December 2019–July 2020	Scopus, Science Direct, and the PubMed database.	49	Cannot provide a specific conclusion.	A negative association between relative humidity and the mortality rate was found.	__	Wind speed: A Positive relationship between wind speed and COVID-19 confirmed cases was also found and the mortality rate of COVID-19 increased with the wind speed. Solar radiation: Negative correlation between solar radiation and COVID-19 cases. Water and wastewater: No indication of the SARS-CoV-2 infection through water; however, the infection risk through water polluted by infected wastewater is still being investigated.	Environmental conditions are a factor in transmitting the virus beyond geographical borders.
6	Mecenas et al. 12	China, Italy, USA, Canada, Japan, France, Germany, Thailand, Singapore, South Korea, Iran, Spain, 2020	2019–March 2020	PubMed, Scopus, Web of Science, Cochrane Library, LILACS, OpenGrey, and Google Scholar	17	Warmer climates are less likely to spread the virus.	Dry conditions were a potent factor in the spread of the virus.	__	__	Warm and wet climates seem to reduce the spread of COVID-19. However, these variables alone could not explain most of the variability in disease transmission. Therefore, the countries most affected by the disease should focus on health policies, even with climates less favorable to the virus.
7	Guo et al. 26	__ 2021	2020	Web of Science	22	Survival rate reduces with the temperature, and virus inactivation generally occurs at 25–30°C.	The COVID-19 virus in the air and on surfaces can be reduced at a relative humidity of 50–70%.	__	Suggests that human coronaviruses, including SARS-CoV-2, display strong winter seasonality.	Virus viability and infectivity are increased at a low temperature of 5°C and reduced at higher temperatures. Virus survival and transmission increase in a dry environment with minimal relative humidity. Also, in a wet condition with high relative humidity, transmission increases, and it is low at intermediate relative humidity.
8	Briz-Redón and Serrano-Aroca 31	Africa, Asia, Europe, North America, South America, World, 2020	May 2020	Web of Science, PubMed, and Google Scholar	61	All the studies considered in this systematic review thus obtained controversial results, and none found clear evidence that a temperature rise reduces case counts of COVID-19. Nonetheless, most studies suggest a negative correlation between COVID-19 and temperature.	Negative correlation between COVID-19 and humidity.	__	Association between COVID-19 and other meteorological factors such as precipitation, radiation, and wind speed has hardly been analyzed, and the findings are unclear	Suggests that weather conditions such as humidity, precipitation, radiation, temperature, and wind speed could play a secondary role in disease transmission. The disparate findings reported seem to indicate that the estimated impact of hot weather on the transmission risk is not large enough to control the pandemic
9	Katoto et al. 28	Europe, Asia, USA, Latin America, 2021	December 2019 and September 2020	PubMed/MEDLINE, Google Scholar, Embase, Web of Science, WHO COVID-19 database, Cochrane Library.	26	__	__	Short term effects: PM2.5: Potential positive association of mortality and susceptibility, but the evidence is uncertain. PM10: Association with mortality is very uncertain, and a positive but uncertain association with susceptibility is reported. NO2: The Association of mortality and susceptibility was contradictory and very uncertain, respectively. O3: Association with mortality was very uncertain, with an uncertain positive association with susceptibility. Long term effects: PM2.5: Association with mortality shows high certainty of being positive. There is also moderately certain evidence of a positive association with the incidence of COVID-19. PM10: Positive association with mortality, leading to an uncertain positive association. NO2: Positively associated with mortality, with a high degree of certainty. O3: Overall association with mortality is very uncertain.	__	Air pollution has adversely influenced the COVID-19-related burden. Short-term and long-term exposures to PM2.5 and long-term exposures to NO2 appear associated with COVID-19. However, studies assessing the effects of acute exposures presented substantial risks of bias.

Discussion

The relationship between meteorological factors and incidence, transmission, and mortality of COVID-19 is complex and inconsistent across studies. The investigation of any possible correlation between meteorological variables and COVID-19 is sophisticated due to a variety of confounding factors, including different healthcare and public health measures across countries, behavioral patterns influenced by climate, and socio-economic status. In this review, by including nine systematic reviews that involved 298 studies, we found evidence of associations between temperature, humidity, air pollution, seasonal variations, wind speed, wastewater, precipitation, and sunlight exposure with COVID-19 transmissibility, incidence, and mortality.

Across the included studies, negative associations were found between temperature and COVID-19 spread and mortality (6/9 studies, 66.6%), implying a decrease in transmission and mortality with increasing temperature. Similarly, seven systematic reviews reported a negative correlation between humidity and transmission or mortality of COVID-19 (7/9 studies, 77.7%). COVID-19 spread was not associated with precipitation (three studies) but was negatively correlated with sunlight or UV radiation (two studies). One study reported that the effect of air pressure and UV radiation on COVID-19 activity was unknown.

Temperature effects

Although the impact of temperature fluctuations on COVID-19 was not demonstrated in two of the included studies (ID: 5, 8), the other studies reported a negative association between temperature and SARS-CoV-2 transmission.^{12,24–27,30} Studies have demonstrated that coronaviruses, including SARS-CoV-2, exhibit temperature sensitivity, with higher temperatures (25°C–30°C) reducing viral viability, while colder temperatures (5°C) enhance stability, and prolong survival on surfaces. ²⁶ Similarly, UV light has been shown to inactivate SARS-CoV-2 by damaging its RNA, with certain wavelengths (e.g., UVC) exhibiting the strongest virucidal effects. ³² Additionally, humidity appears to influence viral transmission, with intermediate relative humidity (50%–70%) reducing airborne viral load, whereas excessively dry or humid conditions may enhance transmission. ²⁶ In a systematic review of 62 ecological studies, Zheng et al., concluded that higher temperatures might reduce the viral activity and infectivity of COVID-19, hence reducing the spread of the virus. ²⁷ The authors noted that in colder weather, individuals tend to engage in more indoor activities, while increased contact and poor ventilation owing to closed windows may lead to a rise in COVID-19 morbidity. Consistent with this, Guo et al., emphasized the winter seasonality of SARS-CoV-2 and stated that the low indoor temperature produced by inadequate heating or excessive ventilation during the winter months may favor viral survival and enhance COVID-19 infection. ²⁶ The authors also noted that the virus is more infectious and viable at temperatures as low as 5°C, whereas it becomes inactive between 25°C and 30°C. Correspondingly, in a systematic review of 23 studies, McClymont and Hu, identified a robust negative association between temperature and COVID-19, noting that the highest incidence of the illness was documented in the temperature range of 0°C–17°C. ¹⁰

Regarding morbidity and mortality of COVID-19, one study conducted in 86 geographical areas across the USA stated that a 1°C increase in ambient temperature was associated with a 6% decrease in COVID-19 mortality at 30-day follow-up. ³³ In a very similar study across 166 countries, Wu et al. showed that temperature was negatively correlated to both daily COVID-19 new cases and mortalities and stated that a 1°C increase in temperature was associated with a 3.08% and 1.19% reduction in daily new cases and new deaths, respectively. ³⁴ One study conducted in Africa noted that with an increase of 1°C in mean daily temperature, the quantity of daily COVID-19 cases dropped by 13.53%. ³⁵ Additionally, Saputra et al., reported a positive association between temperature fluctuations and the daily incidence of COVID-19, suggesting that weather patterns have probably contributed to the pandemic’s spread across Asia. ³⁰ Similarly, Zazouli et al., reported that temperature fluctuations may alter the COVID-19 infection, morbidity, and mortality rates and that a temperature rise might diminish the incidence of the disease. ²⁴ On the other hand, they also stressed that a temperature rise may cause more individuals to spend time outdoors, which might explain why the COVID-19 incidence rate is not lower in the summer.

Moreover, one study in Nigeria found a significant but weak negative correlation between temperature and COVID-19 mortality. ³⁶ Rahman et al., in their huge study among 149 countries, reported a negative association between temperature and mortality in countries with high-income economies. ³⁷ One study conducted in the early months of the pandemic in China among approximately 50,000 new COVID-19 cases stated that for every 1.0 °C increase in temperature, the daily cumulative relative mortality risk reduced by 12.3%. Moreover, they noted that the delayed impacts of the low temperatures are acute and short-term, with the highest risk occurring 5–7 days post-exposure. However, the delayed impacts of high temperatures were found to appear quickly, then reduce rapidly, and increase sharply 15 days post-exposure, mainly emerging as acute and long-term effects. ³⁸ One meta-analysis of 11 studies across 110 countries signified that there was a significant correlation between temperature with COVID-19 death rate and incidence. ³⁹

On the other hand, Paraskevis et al., argued that meteorological factors alone are inadequate to minimize the occurrence of outbreaks in the absence of appropriate public health initiatives. ⁴⁰ Also, Rahman et al., reported an unexpected significant and positive correlation between daytime temperature variation and mortality in the low and middle-income countries. ³⁷ Jüni et al., in their analysis of more than 300,000 COVID-19 cases across 144 geographical areas, found no associations of epidemic growth with latitude and temperature. ⁴¹ Thus, it is crucial to better comprehend the relationship between temperature alterations and COVID-19 in order to develop effective strategies for preventing future outbreaks.

Humidity effects

All studies except for one investigated the possible associations of humidity and COVID-19. In general, seven articles concluded that higher humidity ranges to have a negative impact on COVID-19 in terms of both transmission and/or mortality. Four studies reported negative correlations between COVID-19 and humidity (ID: 3–5, 8). Starke et al. and Rahimi et al., both stated a negative correlation of humidity indices and COVID-19 mortality rates.^25,29 Consistent with their findings, Saputra et al., reported a significant reduction in COVID-19 transmission by higher humidity levels. ³⁰ In line with their findings, one study found that dry conditions and arid locations could be considered as potentiating factors in the spread of SARS-CoV-2, ¹² and a relative humidity of 50%–70% would reduce the levels of SARS-CoV-2 in the air and on surfaces. ²⁶ Similarly, Marzoli et al., reported that environmental factors can impact the virus survival and stated that low temperatures and low humidity levels would prolong the survival of viruses on contaminated surfaces regardless of surface type. The authors also stated that exposure to sunlight significantly decreases the risk of surface transmission. ⁴² Moreover, one similar meta-analysis also noted a small negative correlation (r = − 0.13) between humidity and COVID-19, but it was not statistically significant. ³⁹ McClymont and Hu, also stated that humidity was significantly associated with COVID-19 incidence and found a significant interaction between humidity and temperature. ¹⁰

In addition, Raina et al., proposed that countries with higher COVID-19 prevalence are countries with cold weather and low humidity, which could be a positive factor for transmission and survival of the SARS-COV-2. ⁴³ Wu et al. in their study among more than half a million COVID-19 cases across 166 countries unraveled that a 1% increase in relative humidity was associated with a 0.85% and 0.51% decrease in daily new cases and new deaths, respectively. ³⁴ Similarly, a negative association with COVID-19 daily death counts was observed for relative humidity (r = −0.32), in addition, the authors stated that a 1 unit increase in absolute humidity was associated with decreased COVID-19 death counts. ⁴⁴ Jüni et al., also reported a weak negative association between epidemic growth with relative and absolute humidity.

On the other hand, one study, despite including a few studies in favor of positive correlations between specific humidity ranges and the incidence of COVID-19, could not provide a specific conclusion. ²⁷ Similarly, Chen et al. in their systematic review stated that although most of their included studies revealed that high temperature and humidity can partly decrease the COVID-19 reproduction, morbidity, and mortality, some studies failed to find a significant association. ⁴⁵ Also, Pan et al. in their study among eight countries did not find any significant association between COVID-19 reproductive number and weather conditions, including humidity. ⁴⁶

While many studies reported negative correlations between humidity and COVID-19 incidence and mortality, some conflicting results exist. One critical limitation in interpreting these findings is the potential role of confounders, such as human behavior. Indoor humidity levels are often controlled in public spaces, and higher relative humidity may reduce virus transmission through airborne droplets and fomites. However, some studies failed to account for ventilation quality and indoor conditions, which are essential for understanding the true effect of humidity on COVID-19 transmission.^10,26

Air pollution

The role of air pollution as a possible carrier or risk factor for COVID-19 was expressed by two studies.^24,28 Several studies suggest that air pollutants, particularly PM2.5 and NO₂, exacerbate COVID-19 severity, and mortality. Long-term exposure to air pollution is linked to higher baseline inflammation, which may predispose individuals to severe COVID-19 outcomes. However, further research is required to distinguish the independent effects of air pollution from other urban factors, such as population density and healthcare infrastructure.^28,47 Zazouli et al., came up with the hypothesis that air pollution might aggravate the effect of COVID-19 on health and increase its morbidity. ²⁴ Another study evaluated the air pollution as a short- and long-term effecting factor. ²⁸ Different pollutants were assessed in this study. It was found that PM2.5 could affect mortality in the short term, although the evidence was uncertain. In contrast, in terms of long-term effects, evidence with high certainty showed that PM2.5 exposure could be associated with higher COVID-19 mortality.

Among other pollutants, NO₂ was also reported to be associated with COVID-19 mortality. Although other pollutants, including PM10 and O₃, seem to be associated with higher mortality rates, the evidence was uncertain. ²⁸ Copat et al., in their systematic review of 15 studies concluded that, although the potential impact of airborne SARS-CoV-2 exposure has not been understood, and undoubtedly more research is needed to increase scientific evidence, current major findings signify the contribution of PM2.5 and NO₂ as triggering and aggravating factors for COVID-19 spread and lethality, and to a less extent also PM10. ⁴⁷

Moreover, there was one large systematic review of 116 articles that investigated the possible relations between ambient air pollutants, such as PM2.5, PM10, or gases (NO₂, NOX, O₃, CO, SO₂), and COVID-19 spread, non-fatal severity, and deaths. This large review stated that among all evaluations, 52.7% of all evaluations found that COVID-19 spread was positively and significantly associated with higher air pollution. Also, the corresponding percentage for COVID-19 mortality and air pollution exposure was 48.1%, while, for non-fatal severity, 41.2% of articles had reported positive and significant associations. Hernandez Carballo et al., also found that PM2.5 and NO₂ are most frequently associated with higher mortalities, in contrast to PM10 and O₃, which did not show any statistically significant relationship with COVID-19 deaths. ⁴⁸

It can be concluded that the air pollutants might be associated with higher COVID-19 morbidity, mortality, and poorer health outcomes. However, further investigation is needed for a definitive conclusion because other meteorological and environmental factors, including particle size, weather, wind speed, humidity, and temperature, can affect air pollution.

Seasonal variation and other factors (e.g., wind speed, wastewater, precipitation, and sunlight)

Seasonal variation, which was assessed by multiple studies, is another factor that could affect the transmission and activity of COVID-19. ³⁰ There is evidence that winter seasonality plays a role in COVID-19 transmission, but the exact mechanisms remain uncertain. Some studies suggest that colder temperatures and decreased UV exposure contribute to increased virus stability, while behavioral changes in winter (e.g., increased indoor crowding) likely play a more significant role. ²⁶ Similarly, the impact of wind speed remains unclear, with conflicting evidence suggesting both positive and negative associations with COVID-19 transmission. Future studies should standardize methodologies to ensure comparability across geographic regions.^28,35,47 Although some studies did not make conclusive comments or reported unclear results in terms of wind speed, Rahimi et al. showed that COVID-19 mortality could be accentuated with increased wind speed. ²⁹ Adekunle et al. found a positive association between COVID-19 cases and wind speed, and reported that a 1% increase in wind speed average was positively and significantly associated with a 11.21% increase in COVID-19 cases. ³⁵ In stark contrast, two studies conducted in Taiwan and Saudi Arabia found a significant negative correlation between wind speed and COVID-19, and reported that increasing wind speed was associated with a decline in COVID-19 incidence.^49,50

Sewage was known to be responsible for the pollution of the environment and the distribution of SARS-CoV-2 in one study. ²⁴ This is in contrast to the Rahimi et al., findings about the wastewater, where no evidence of risk of infection with COVID-19 through the water was reported. ²⁹ In that regard, one systematic review of 92 studies in 34 countries reported an overall sample positivity of 29.2%. The authors stated that wastewater findings preceded the confirmed cases by up to 63 days, and reported an association between wastewater viral load with crude numbers of community cases. ⁵¹

Regarding precipitation, many studies found no significant correlation between precipitation and COVID-19,^52,53 while two studies reported significant negative correlations between rainfall and COVID-19 new cases. Additionally, one study indicated that daily cases of COVID-19 increased when there was between 1.27 and 1.74 inches of rainfall and decreased with rainfalls more than 1.77 inches in a month.^54,55

Also, seasonal variation due to changes the temperature and rainy seasons was shown to be an important factor in COVID-19 infectivity. According to some studies, the incidence of COVID-19 decreased with sunlight exposure and warm climates.^29,30 Guo et al., showed winter seasonality for COVID-19 infection. ²⁶ Regarding sunlight effects, which consequently is associated with higher vitamin D levels, one systematic review of the nine studies showed that COVID-19 infection, outcomes, and mortality were correlated with vitamin D levels. Furthermore, it was stated that blood vitamin D status can determine COVID-19 infection risk, severity, and associated mortalities. Keeping normal Vitamin D levels through supplementation or adequate sunlight exposure, proposed as a key element to cope with the pandemic. ³² Similarly, Pereira et al. in their meta-analysis mentioned that more vitamin D deficiency was observed among severe COVID-19 cases compared with mild cases. ⁵⁶ Thus, it can be proposed that societies with more regular sunlight and UV radiation exposure would have less vitamin D deficiency and, consequently, lower COVID-19 mortality rates.

In contrast, there are also other studies that reported an unknown or even no association between solar radiation and COVID-19 activity.^27,31 Synthesizing all this information, it could be hypothesized that higher temperatures and warm weather have a negative correlation with the incidence of COVID-19, or at least being associated with milder symptoms. However, further investigation regarding environmental temperature and COVID-19 is necessary for precise results.

Strengths and limitations

One strength of our systematic review was the thorough methodology used, which is consistent with the PRISMA checklist; briefly, the screening, data extraction, and quality assessment were performed by two independent reviewers. Our study provides the ability to identify and compare probable associations between environmental variables (i.e., temperature, humidity, air pollution, seasonal changes, wind speed, precipitation, and UV radiation) and COVID-19 incidence and mortality. Additionally, we highlight the necessity of controlling for confounders in future research. Many existing studies lack adjustments for key behavioral and environmental factors, which limits the reliability of reported associations. Addressing these issues will improve the robustness of future research.^10,26 In addition, it provides potential weather-associated risk factors and predictors for the COVID-19 pandemic. Moreover, the present systematic review included a large number of global studies (n = 298) and countries across a wide geographical range, and involving a wide range of climates. However, our study had some limitations, which will be acknowledged. It was difficult to assess long-term patterns, possible annual seasonality, and climate patterns due to the short period since the beginning of the pandemic. Moreover, many confounding factors were involved as strict measures (e.g., quarantines, lockdowns, and travel restrictions) were taken that significantly impacted the COVID-19 incidence and transmission, and would confound the COVID-19 incidence, thus associations with weather variables. Also, the association between temperature and humidity and COVID-19 may be impacted by the seasonality of COVID-19 and may be coincidental. In addition, the availability and quality of testing may have affected the precision and accuracy of test results and might have caused large discrepancies between the true number of cases and detected cases, specifically in countries with limited healthcare resources. Another considerable limitation across many of the included studies is the inconsistent control for crucial confounding factors. While associations between meteorological variables and COVID-19 are reported, these findings may be significantly influenced by, or even entirely attributable to, alternative mechanisms such as varying public health interventions (e.g. lockdowns, mask mandates), population density, healthcare access, testing strategies, and behavioral changes influenced by socio-economic factors, which were often not adequately accounted for in the original research. Therefore, the observed correlations should be interpreted with caution, and future research rigorously addressing these confounders is essential to establish more definitive causal relationships.

Policy implications

The findings of this review have several implications for public health policy. Understanding the seasonality of COVID-19 can inform preparedness strategies, particularly in regions experiencing extreme temperature fluctuations. For instance, higher transmission rates during colder months highlight the need for targeted indoor ventilation policies and reinforcement of mask mandates in enclosed spaces. Furthermore, considering the potential role of air pollution in exacerbating COVID-19 severity, governments may need to integrate air quality management into pandemic response plans. ²⁸ Lastly, the demonstrated impact of UV light on virus inactivation suggests that controlled application of UV disinfection technologies in healthcare and public settings could be explored as an adjunct mitigation strategy.

Conclusion

This systematic review of the associations between climate or meteorological factors and COVID-19 proposes that weather conditions such as temperature, humidity, air pollution, seasonal changes, wind speed, precipitation, and UV radiation can be considered as contributing factors to COVID-19 transmission and mortality, specifically temperature and humidity. A negative correlation between temperature and humidity and COVID-19 incidence or mortality was reported in most studies, which indicates that an increase in either temperature or humidity would cause a reduction in COVID-19 new cases or death counts. However, there were some contradictory findings across studies regarding the association between temperature or humidity and COVID-19. Temperature and humidity ranges for virus infectivity and inactivation were proposed. Air pollution was suggested as a possible short and long-term risk factor for COVID-19; however, generally, the evidence was uncertain and inconsistent. Although, most studies reported that they could not provide any definite conclusion or had unclear results in terms of wind speed effects, Wind speed was proposed as a possible accentuating factor for COVID-19 mortality, although most studies could not reach a definite conclusion on this matter. Environmental pollution by wastewater was found to be a possible contributing factor for SARS-CoV-2 distribution; however, these findings were inconsistent and inconclusive across studies. Also, a few studies suggested that the incidence of COVID-19 would decrease with sunlight exposure and warm climates, and proposed winter seasonality for COVID-19 infection. Therefore, further investigation and analysis of the COVID-19 pandemic are crucial in understanding the effects of weather on its transmission and mortality. Consequently, this information would help control future pandemics and take vast public health measures.