Researchers discovered a 1 percent decrease in humidity could increase the number of COVID-19 cases by 6 percent


A study conducted in Sydney during the early epidemic stage of COVID-19 has found an association between lower humidity and an increase in locally acquired positive cases. Researchers discovered a 1 percent decrease in humidity could increase the number of COVID-19 cases by 6 percent.

The research led by Professor Michael Ward, an epidemiologist in the Sydney School of Veterinary Science at the University of Sydney, and two researchers from our partner institution Fudan University School of Public Health in Shanghai, China, is the first peer-reviewed study of a relationship between climate and COVID-19 in the southern hemisphere.

“COVID-19 is likely to be a seasonal disease that recurs in periods of lower humidity. We need to be thinking if it’s wintertime, it could be COVID-19 time,” said Professor Ward.

The study is published today in Transboundary and Emerging Diseases.

Further studies – including during winter in the southern hemisphere – are needed to determine how this relationship works and the extent to which it drives COVID-19 case notification rates.

Previous research has identified a link between climate and occurrence of SARS-CoV cases in Hong Kong and China, and MERS-CoV cases in Saudi Arabia, and a recent study on the COVID-19 outbreak in China found an association between transmission and daily temperature and relative humidity.

“The pandemic in China, Europe and North America happened in winter so we were interested to see if the association between COVID-19 cases and climate was different in Australia in late summer and early autumn,” Professor Ward said.

“When it comes to climate, we found that lower humidity is the main driver here, rather than colder temperatures,” Professor Ward said.

“It means we may see an increased risk in winter here, when we have a drop in humidity. But in the northern hemisphere, in areas with lower humidity or during periods when humidity drops, there might be a risk even during the summer months. So vigilance must be maintained.”

Why humidity matters

Professor Ward said there are biological reasons why humidity matters in transmission of airborne viruses.

“When the humidity is lower, the air is drier and it makes the aerosols smaller,” he said. “When you sneeze and cough those smaller infectious aerosols can stay suspended in the air for longer.

That increases the exposure for other people.

When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker.”


Professor Ward and his team studied 749 locally acquired cases of COVID-19 – mostly in the Greater Sydney area of the state of New South Wales – between February 26 and March 31.

The team matched the patients’ postcodes with the nearest weather observation station and studied the rainfall, temperature and humidity for the period January to March 2020.

The study found lower humidity was associated with an increased case notifications; a reduction in relative humidity of 1 percent was predicted to be associated with an increase of COVID-19 cases by 6 percent.

“This means we need to be careful coming into a dry winter,” Professor Ward said, adding that the average humidity in Sydney is lowest in August.

“Even though the cases of COVID-19 have gone down in Australia, we still need to be vigilant and public health systems need to be aware of potentially increased risk when we are in a period of low humidity,” Professor Ward said. “Ongoing testing and surveillance remain critical as we enter the winter months, when conditions may favour coronavirus spread.”

Further research

Professor Ward said the study was limited to cases contracted in the summer months mostly in and around Sydney, so further research is needed in the months to come and further afield. In winter, cooler temperatures may be also be a factor.

The World Health Organization declared the COVID-19 epidemic a public health emergency of international concern on 30 January 2020 (WHO, 2020a). Respiratory viruses, such as SARS-CoV and MERS-CoV, contained in infectious droplets and body fluids are capable of contaminating the human conjunctival epithelium and inducing complications in infected patients, thus leading to respiratory infection (Belser et al., 2013; Olofsson et al., 2005).

The transmission of SARS-CoV-2 between humans may occur by three routes (NHC, 2020a): 1) direct transmission via inhalation of respiratory droplets (coughs or sneezes by infected patients in close proximity); 2) contact transmission through touch of a surface or object contaminated with the virus; and 3) aerosol transmission in confined spaces. An early study by Liu et al. (2020) suggests an even more severe transmissibility than SARS-CoV (Liu et al., 2020).

There is significant evidence that air pollution is associated with premature mortality (Lelieveld et al., 2015; Giannadaki et al., 2014) and adverse health effects (West et al., 2013; Hirabayashi and Nowak, 2016). A global estimate showed that 4.3 million deaths occurred as a result of deteriorated air quality (Lelieveld, 2017; Cohen et al., 2017).

Elevated nitrogen oxides (NOx) and particulate matter (PM) concentrations have been linked to increased incidence rates of cardiovascular and pulmonary diseases, asthma, diabetes and cancers (Shiraiwa et al., 2017; Hertel et al., 2013).

Specifically, air pollution has been linked to virus-induced diseases, such as influenza (Chen et al., 2010; Thach et al., 2010), pneumonia and acute lower respiratory infections (Horne et al., 2018; Glass and Rosenthal, 2018), and severe acute respiratory syndrome (SARS) (Cui et al., 2003).

A positive association between air quality and SARS case fatality was identified by Cui et al. (2003). As a major air pollutant, particulate matter (PM) is capable of remaining airborne for a long period (Cowling et al., 2013; Kim et al., 2015).

Infectious virus and viral RNA can be detected on particles with aerodynamic diameters larger and smaller than 5 μm (Milton et al., 2013; Lindsley et al., 2010). PM of 5 μm or less in diameter attached with viruses can be inhaled and penetrated deep into the respiratory tract and to the alveolar region (30% penetration for 5 μm particles). Inside the human body, viral agents attached on the PM can be delivered directly to the respiratory epithelial cells and translocated to other organs (Nemmar, 2004; Tellier, 2009), thus inducing infections and various health effects.

Particularly, airborne PM2.5 (PM with aerodynamic diameter ≤ 2.5 μm) has been reported to be associated with daily human influenza cases (Lindsley et al., 2010; Liang et al., 2014;) and respiratory syncytial virus infection (Vandini et al., 2013; Nenna et al., 2017).

Moreover, SARS mortality was found to be positively correlated with PM with aerodynamic diameter smaller than 10 μm (PM10). Additionally, meteorological conditions, such as temperature and humidity are associated with the spread of numerous viral diseases, such as influenza and respiratory syncytial virus (Bloom-Feshbach et al., 2013; Lowen et al., 2007), SARS and MERS (Lin et al., 2006; Gardner et al., 2019).

Epidemiological studies have shown that lower temperature may increase the risk of transmission for both SARS and MERS (Lin et al., 2006; Gardner et al., 2019), and infection with MERS-CoV is more likely to occur under dry conditions (Gardner et al., 2019).

Although much more about COVID-19 remains to be learned, the causal pathogen, SARS-CoV-2 belongs to the same virus family as SARS-CoV and MERS-CoV, and all three of these coronaviruses have been identified to be capable of airborne transmission (Zhou et al., 2020a; Yu et al., 2004; Zumla and Hui, 2014).

Moreover, the transmission of SARS and MERS has been associated with air quality and meteorological conditions (Cui et al., 2003; Lin et al., 2006; Gardner et al., 2019). Therefore, it is reasonable to speculate that environmental and meteorological factors might affect the spread of COVID-19.

The spread of the COVID-19 epidemic has significantly declined in some counties, e.g., in China, due to unprecedented nationwide interventions. However, the COVID-19 outbreak shows no signs of slowing down from a global perspective. A more comprehensive understanding of COVID-19, including the possible potential impacts of environmental factors, would be of significance for containing its spread. Therefore, this study focuses on analyzing the association between the air quality index (AQI) and the confirmed cases of COVID-19 and investigating the effect of temperature and humidity on the AQI – COVID-19-confirmed case association.

University of Sydney


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