To the Editor—I read with great interest a recent article by Cheng et al^{Reference Cheng, Wong and Chan1} that examined the contamination status of air and environmental surfaces with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in airborne infection isolation rooms occupied by single symptomatic and asymptomatic patients with coronavirus disease 2019 (COVID-19). In this study, SARS-CoV-2 RNA was not detected in air samples and was infrequently detected in environmental samples (eg, patients’ mobile phones, bedrail, and toilet door handle) taken before daily cleaning and disinfection of the isolation rooms. Cheng et al^{Reference Cheng, Wong and Chan1} concluded that rigorous hand hygiene, environmental cleaning and disinfection, and appropriate use of surgical masks were sufficient for healthcare infection prevention and control, except during aerosol-generating procedures. Other studies in different countries also investigated both air and environmental contamination with this novel coronavirus in healthcare settings, with variable contamination status findings (Table 1).^{Reference Cheng, Wong and Chan1–Reference Kanamori, Weber and Rutala12}

Table 1.

Air and Environmental Contamination With Severe Acute Respiratory Syndrome Coronavirus 2 in Healthcare Settings

Note. AGP, aerosol-generating procedure; COVID-19, coronavirus disease 2019; CPAP, continuous positive airway pressure; ICU, intensive care unit; NA, not applicable; National Institute of Occupational Safety and Health, NIOSH; PPE, personal protective equipment; RT-PCR, reverse transcription polymerase chain reaction; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

These studies assessed air and environmental contamination with SARS-CoV-2 by either reverse-transcription polymerase chain reaction (RT-PCR) or viral culture or both. Detecting SARS-CoV-2 RNA in air or aerosol samples does not verify the presence of viable virus. Furthermore, methods of air sampling for data collection, analysis, and interpretation, including air sampler type, particle size, air volume, airflow rate, and sampling time and place are not standardized.^{Reference Kanamori, Rutala, Sickbert-Bennett and Weber13} Zhou et al^{Reference Zhou, Otter and Price11} reported that viral culture did not show viable SARS-CoV-2, even though 2 of 31 air samples (6.4%) and 23 of 218 surface samples (10.6%) were positive for its RNA (cycle threshold [Ct] value > 30 in all samples). In an experimental study, they allowed various dilutions of SARS-CoV-2 to dry on steel or plastic surfaces and found culturable SARS-CoV-2 in dried inoculum (Ct value < 30). After 3 days of culture, Santarpia et al^{Reference Santarpia, Rivera and Herrera9} observed viral proteins by immunofluorescence in a hallway sample, although they did not confirm cultivation of SARS-CoV-2.

Studies of air and environmental surfaces found that if air samples were positive for SARS-CoV-2 RNA, environmental surface samples were also (Table 1).^{Reference Chia, Coleman and Tan2,Reference Guo, Wang and Zhang3,Reference Lei, Ye and Liu5,Reference Razzini, Castrica and Menchetti8,Reference Santarpia, Rivera and Herrera9,Reference Zhou, Otter and Price11} Air samples taken <1 m from a patient receiving high-flow nasal cannula oxygen therapy were contaminated, but air and surface contamination levels were lower in intensive care units probably because of the use of closed-circuit ventilation systems.^{Reference Zhou, Otter and Price11} Several studies have shown that even when air samples were negative for SARS-CoV-2 RNA, environmental samples from air outlets were positive.^{Reference Cheng, Wong and Chan1,Reference Ong, Tan and Chia7,Reference Wei, Lin and Duan10} Wei et al^{Reference Wei, Lin and Duan10} reported that surfaces in patient rooms with air exhaust outlets that tested positive for SARS-CoV-2 RNA were frequently contaminated (26.7%–95.7%), suggesting that small virus-laden particles are present around patients. In one study, environmental samples collected after cleaning and disinfection, and all air samples except for those from air exhaust outlets, were negative in RT-PCR, although the small volumes of the samples may have affected these results.^{Reference Ong, Tan and Chia7}

Person-to-person SARS-CoV-2 transmission occurs primarily via respiratory droplets and contact, but some scientists suggested that airborne transmission (microdroplets or aerosols) also occurs.^{Reference Morawska and Milton14} Several studies have shown that air samples were positive for SARS-CoV-2 RNA in isolation rooms with 12 air changes per hour.^{Reference Chia, Coleman and Tan2,Reference Guo, Wang and Zhang3,Reference Santarpia, Rivera and Herrera9} Viral RNA was detected in the air within 4 m of a patient, indicating possible aerosol transmission of SARS-CoV-2.^{Reference Guo, Wang and Zhang3} Samples from the air around severely ill patients treated with aerosol-generating procedures were likely to be positive for SARS-CoV-2 RNA.^{Reference Lei, Ye and Liu5,Reference Razzini, Castrica and Menchetti8,Reference Zhou, Otter and Price11} However, the positivity rates of air samples collected around patients who did not receive aerosol-generating procedures have been discordant,^{Reference Cheng, Wong and Chan1,Reference Chia, Coleman and Tan2,Reference Ong, Tan and Chia7,Reference Santarpia, Rivera and Herrera9,Reference Wei, Lin and Duan10} while environmental surface samples were positive for SARS-CoV-2 RNA in all of these studies. Tang et al^{Reference Tang, Mao and Jones15} also reviewed the scientific evidence for aerosol transmission of SARS-CoV-2 and potential control measures in various situations and populations, highlighting that healthcare personnel are at high risk for aerosol transmission of SARS-CoV-2 in the closed hospital environment.^{Reference Tang, Mao and Jones15} Viral aerosol particles can be generated by mildly ill patients without a cough, leading to extensive environmental and potential aerosol contamination with SARS-CoV-2; however, no cases of COVID-19 were documented in healthcare personnel who took airborne precautions.^{Reference Santarpia, Rivera and Herrera9} On this basis, healthcare personnel should implement airborne precautions when performing aerosol-generating procedures in patients with COVID-19.