According to research summarized by ASHRAE, there are three ways that low humidity can makes transmission of respiratory infections more likely. While this research is not specific to COVID-19, it is applicable to respiratory infections in general. First, large infectious particles can shrink in dry air to become droplet nuclei, which are dormant but still infectious and can travel long distances. Second, many viruses are most viable in low humidity. Third, there is evidence that ambient humidity below 40% lowers immune system defenses, in part by impairing mucus membrane barriers. Detailed analysis of many of these issues can be found in "Seasonality of Respiratory Infections."