Generic image illustrating an air filter Share this article Press release issued: 29 April 2021 There is an important absence of evidence regarding the effectiveness of a potentially cost-efficient intervention to prevent indoor transmission of respiratory infections, including COVID-19, warns a study by researchers at the University of Bristol. Respiratory infections such as coughs, colds, and influenza, are common in all age groups, and can be either viral or bacterial. Bacteria and viruses can become airborne via talking, coughing or sneezing. The current global coronavirus (COVID-19) pandemic is also spread primarily by airborne droplets, and to date has led to over three million deaths worldwide.