Air contamination of households versus hospital inpatient rooms occupied by SARS-CoV-2 positive patients.

Households are settings with some of the highest COVID-19 secondary-attack-rates. We compared the air contamination in hospital rooms versus households of COVID-19 patients. Inpatient air-samples were only positive at 0.3 meters from patients. Household air samples were positive even without a COVID-19 patient in the proximity to the air sampler.


Introduction
SARS-CoV-2 is transmitted primarily by respiratory droplets and contact with contaminated surfaces and fomites. 1 Airborne transmission is still a controversial topic among the scientific community. A few studies have successfully identified SARS-CoV-2 in the air of hospital rooms using real-time reverse transcriptase polymerase chain reaction (RT-PCR) or viral cultures. [2][3][4][5] However, air contamination in households has yet to be characterized. Households are settings with high secondary attack rates; members of the same household have been shown to experience up to ten times greater risk of COVID-19 than other contacts (i.e., healthcare workers, workplace contacts, and non-household contacts). [6][7][8] Understanding the degree of air contamination in household settings would help us tailor prevention interventions in these high risk settings. To address this knowledge gap, our study aimed to characterize and compare the presence of SARS-CoV-2 in air samples obtained in household settings against air samples obtained in inpatient rooms both selected based on the presence of SARS-CoV-2 positive patients.

Methods
This study was performed at Froedtert Memorial Lutheran Hospital, a 607-bed academic medical center affiliated with the Medical College of Wisconsin. This inpatient facility has 6 intensive care units (ICU) with 150 ICU beds. During the pandemic, a few units were designated for cohorting SARS-CoV-2 positive patients, including the medical ICU, the cardiovascular ICU, and a couple of general medical/surgical units. All these units were set to at least 6 air changes per hour and at negative pressure relative to the hallways.
A convenience sample of rooms was selected based on the presence of patients with positive SARS-CoV-2 RT-PCR test. Households were identified based on the presence of a at least one household member with symptoms compatible with COVID-19 and at least one positive SARS-CoV-2 RT-PCR test. All subjects had to have a positive SARS-CoV-2 test in the preceding 7 days from air sampling.
Air sampling and molecular testing: Air samples were collected using the Sartorius MD8 airscan sampling device (SartoriusAG, Germany) with sterile gelatin filters (80 mm in diameter and 3 μm pore size (SartoriusAG, Germany). Briefly, the air sampler was positioned 0.305-1.83 m from the patient's head to collect from 1,000 L to 4,000 L (50 L/min). We evaluated shorter distances and higher volumes until we were able to detect SARS-CoV-2 in air samples. For samples that obtained 4,000 liters, two air samplers were used concomitantly for 40 minutes (50 L/min; 2000 L each). Gelatin filters were placed in 6 mL of viral transport media (VTM) (Remel M4RT, ThermoFisher, Lenexa, KS). If two air samplers were used concomitantly to achieve 4,000 L, then both gelatin membranes were placed in a single container with 6 mL of VTM. These plates were incubated at 37 o C for one minute to allow the gelatin filter to dissolve. This mixture was then vortexed and centrifuged at 13,000×g for 1 minute, and 1 mL of the supernatant was used for nucleic acid extraction. Nucleic acid Downloaded from https://www.cambridge.org/core. 18 Feb 2021 at 08:37:23, subject to the Cambridge Core terms of use. extraction and RT-PCR was performed on the Cobas 6800 (Roche, Indianapolis, IN) per the manufacturers EUA approved product insert. For patient specimens, combined nasopharyngeal and oropharyngeal swabs were collected from each patient and both the nasopharyngeal and oropharyngeal swabs were placed into 3 mL VTM (Copan, Murrieta, CA). RT-PCR was performed to detect the presence of SARS-CoV-2 on each sample using the Cobas 6800 per the manufacturers EUA approved product insert.
Hospital air samples: Out of 15 patient rooms sampled, 14 were located in COVID units (5 in ICUs and 9 in general medicine wards) and 1 on a non-COVID unit. All rooms were set to have at least 6 air changes per hour, with negative pressure relative to the hallway, and their median square size was 260.1 m 2 (range 89.2-260.1). Out of 15 air samples, 7 sampled 1,000 liters (ranging from 1.83 to 0.91 meters from the patient), being all of these samples negative.  Table).

Discussion
In this study, household samples were eight times more likely to test positive for the virus than inpatient samples. Inpatient rooms only tested positive when the volume of air sampled was quadrupled and the distance between air samplers and patients was minimal. Thus, these positive results may represent contaminated respiratory droplets being expelled by patients Downloaded from https://www.cambridge.org/core. 18 Feb 2021 at 08:37:23, subject to the Cambridge Core terms of use. rather than actual air contamination. Given that room ventilation (i.e., air changes per hour) was the main difference between these settings, our findings may suggest that the degree of ventilation in a room is more important determining the degree of air contamination than the acuity of illness that a SARS-CoV-2 patient may be experiencing. Previous studies have characterized the air contamination in inpatient areas with a wide range of findings between 1.3% to 63.2% [2][3][4][5] ; however, the viability of the virus in air samples is still controversial. To the best of our knowledge, this is the first report of air contamination by SARS-CoV-2 within household settings.
Our study had a small sample size, used convenience samples, and did not perform viral cultures. Further, we did not measure the temperature or humidity of the rooms, which are environmental variables that may potentially impact the viability of the virus. In addition, we obtained more air samples per household than per inpatient room; therefore, there we increased the likelihood of detecting positive samples in households. Despite of our limitations, the preliminary findings presented suggest that household settings may have high degree of air contamination, signaling a major impact of room ventilation on this outcome.
Future studies should characterize the variables determining the degree of air contamination in households and explore innovative ways to ameliorate this, especially in crowded households without access to natural ventilation.