Dear Sir, We read with interest a recent article which modelled the risk to dental health care workers of various airborne pathogens . This paper used Carbon Dioxide (CO2) concentration as an indicator of air quality and ventilation, reasoning that CO2 ;levels provide a measure of exposure to exhaled breath . Reassuringly, with high indoor air quality, and a filtering facepiece-2 (FFP2) mask, infection transmission probability was estimated at 0% for severe acute respiratory syndrome coronavirus (SARS-CoV). CO2 concentration (i.e. air quality and ventilation) had the biggest influence on estimated transmission risk, much more so than respiratory protection from masks. With “medium” air quality the estimated transmission probability remained over 20% regardless of respiratory protection (masks) and patient infectivity. It should be noted that these data were based on SARS-CoV-1, but SARS-CoV-2 is expected to behave similarly. Using CO2 concentration is a useful, quick and inexpensive measure to assess air quality in indoor spaces and may be of use to readers. High air quality is defined as < 800 ppm CO2 . We used a calibrated CO2 meter (Extech CO240; Nashua, NH, USA; £200) to assess four indoor environments within Newcastle dental school and hospital, with known air exchange rates. Two people sat (physically distanced) in each space for 30 minutes, allowing CO2 to accumulate, before leaving the area whilst measurement continued. We found that the increase in CO2 was minimal in most environments, with air quality remaining high (i.e. < 800 ppm CO2). In one unventilated, windowless surgery, levels did reach medium quality for a short period. This highlights the potential value of this simple technique in identifying areas with poorer ventilation. We next repeated the measurements with any windows open, and found that air quality and ventilation improved further; this was close to CO2 levels found outdoors. In conclusion, measuring CO2 concentration may be a useful way to measure indoor air quality and opening a window is a powerful and simple way to improve ventilation. Nisha Patel, Ciara Docherty, James Allison, Graham Walton, Ben Cole, Justin Durham, Nick Jakubovics and Richard Holliday School of Dental Sciences, Newcastle University Newcastle Hospitals NHS Foundation Trust Correspondence to firstname.lastname@example.org References 1. Zemouri C, Awad SF, Volgenant CM, Crielaard W, Laheij AM, de Soet JJ. Modeling of the Transmission of Coronaviruses, Measles Virus, Influenza Virus, Mycobacterium tuberculosis, and Legionella pneumophila in Dental Clinics. J Dent Res 2020; 2:0022034520940288. DOI: https://doi.org/10.1177/0022034520940288. 2. Rudnick SN, Milton DK. Risk of indoor airborne infection transmission estimated from carbon dioxide concentration. Indoor air. 2003; 13(3): 237-45. DOI: https://doi.org/10.1034/j.1600-0668.2003.00189.x 3. Kukadia V, Upton S. Ensuring good indoor air quality in buildings. BRE Group. Ensuring Good Indoor Air Quality in Buildings. 2019. Available at https://www.bregroup.com/bretrust/wp-content/uploads/sites/12/2019/03/Ensuring-Good-IAQ-in-Buildings-Trust-report_compressed-2.pdf (accessed July 2020).