Direct estimates of absolute ventilation and estimated Mycobacterium tuberculosis transmission risk in clinics in South Africa

Abstract

Healthcare facilities are important sites for the transmission of pathogens spread via bioaerosols, such as Mycobacterium tuberculosis. Natural ventilation can play an important role in reducing this transmission. We aimed to measure rates of natural ventilation in clinics in KwaZulu-Natal and Western Cape provinces, South Africa, then use these measurements to estimate Mycobacterium tuberculosis transmission risk. We measured ventilation in clinic spaces using a tracer-gas release method. In spaces where this was not possible, we estimated ventilation using data on indoor and outdoor carbon dioxide levels. Ventilation was measured i) under usual conditions and ii) with all windows and doors fully open. Under various assumptions about infectiousness and duration of exposure, measured absolute ventilation rates were related to risk of Mycobacterium tuberculosis transmission using the Wells-Riley Equation. In 2019, we obtained ventilation measurements in 33 clinical spaces in 10 clinics: 13 consultation rooms, 16 waiting areas and 4 other clinical spaces. Under usual conditions, the absolute ventilation rate was much higher in waiting rooms (median 1769 m3/hr, range 338–4815 m3/hr) than in consultation rooms (median 197 m3/hr, range 0–1451 m3/hr). When compared with usual conditions, fully opening existing doors and windows resulted in a median two-fold increase in ventilation. Using standard assumptions about infectiousness, we estimated that a health worker would have a 24.8% annual risk of becoming infected with Mycobacterium tuberculosis, and that a patient would have an 0.1% risk of becoming infected per visit. Opening existing doors and windows and rearranging patient pathways to preferentially use better ventilated clinic spaces result in important reductions in Mycobacterium tuberculosis transmission risk. However, unless combined with other tuberculosis infection prevention and control interventions, these changes are insufficient to reduce risk to health workers, and other highly exposed individuals, to acceptable levels.

Time to change the way we think about tuberculosis infection prevention and control in health facilities: insights from recent research

“…Using a combination of carbon dioxide (CO 2 ) release experiments 41 and paired indoor–outdoor CO 2 measurements, 42 we directly measured the absolute ventilation rate in 33 clinical spaces across 10 primary healthcare facilities. 43 We found marked variation in natural ventilation between spaces, with the least-ventilated spaces being consultation rooms where doors and windows had been closed to allow air-conditioning units to work. 43 Fully opening existing doors and windows led, on average, to a twofold increase in natural ventilation.…”

“… 43 We found marked variation in natural ventilation between spaces, with the least-ventilated spaces being consultation rooms where doors and windows had been closed to allow air-conditioning units to work. 43 Fully opening existing doors and windows led, on average, to a twofold increase in natural ventilation. 43 …”

“… 43 Fully opening existing doors and windows led, on average, to a twofold increase in natural ventilation. 43 …”

Airborne transmission risks of tuberculosis and COVID-19 in schools in South Africa, Switzerland, and Tanzania: Modeling of environmental data

“…Several studies have modeled transmission risks of respiratory infections in indoor settings within countries [ 15 , 21 – 23 , 30 , 57 ]. However, to our knowledge, comparisons of transmission risks in schools between countries are lacking.…”

“…The ventilation (or replacement) rate can be monitored using indoor carbon dioxide (CO 2 ) as a tracer gas [20]. Ventilation rate is an important parameter in modeling the risk of airborne infection [15,[21][22][23].…”