Water droplets containing the SARS-CoV-2 virus, responsible for coronavirus 2019 transmission, were introduced into a controlled-temperature and -humidity chamber. The SARS-CoV-2 virus with green fluorescent protein tag in droplets was used to infect Caco-2 cells, with viability assessed through flow cytometry and microscopic counting. Whereas temperature fluctuations within typical indoor ranges (20°C–30°C) had minimal impact, we observed a notable decrease in infection rate as the surrounding air’s relative humidity increased. By investigating humidity levels between 20% and 70%, we identified a threshold of ≥40% relative humidity as most effective in diminishing SARS-CoV-2 infectivity. We also found that damage of the viral proteins under high relative humidity may be responsible for the decrease in their activity. This outcome supports previous research demonstrating a rise in the concentration of reactive oxygen species within water droplets with elevated relative humidity.

Disinfectants are important for arresting the spread of pathogens in the environment. Frequently used disinfectants are often incompatible with certain surfaces, expensive and can produce hazardous by-products. We report that micron-sized water droplets can act as an effective disinfectant, which were formed by spraying pure bulk water with coaxial nebulizing airflow. Spraying for 20 min onto Escherichia coli and Salmonella typhimurium on stainless-steel discs caused inactivation of over 98% of the bacteria. Control experiments resulted in less than 10% inactivation (water stream only and gas only) and 55% inactivation with 3% hydrogen peroxide. Experiments have shown that cell death results from cell wall destruction. We suggest that the combined action of reactive oxygen species present in water droplets (but not in bulk water) along with the droplet surface charge is responsible for the observed bactericidal activity.