Air Purifiers and SARS-CoV-2 Virus: Analysis of HEPA Filters as a Surveillance Method

    Researchers from Castilla-La Mancha lead the development of a detection method for the SARS-CoV-2 virus in safe environments, based on the analysis of the HEPA filters contained in air purifiers, which would reduce the economic costs of individual analysis and speed up the diagnostic process of COVID-19 infection.


    In March 2020, the World Health Organization (WHO) officially declared the global pandemic COVID-19, a highly transmissible viral infection caused by the coronavirus. Severe Acute Respiratory Syndrome coronavirus 2, better known as COVID-2. In Spain, 7 months after making the pandemic official and a long and rigorous confinement, schools and nurseries reopened their doors, always under strict sanitary measures to avoid possible outbreaks and infections. Among these measures were frequent hand washing and the use of hydroalcoholic gel, periodic cleaning of surfaces, the physical distance between people of at least 1,5 m and increased ventilation of interior spaces by opening windows and doors.

    Most of the measures are easy to carry out, except one, interior ventilation, which in many cases depends on environmental conditions and the structural configuration of the building itself, which would mean an added difficulty in achieving good levels of ventilation at all times. It is here where the portable air purifiers come into play, becoming necessary and essential in those interiors where ventilation is non-existent or scarce, such as the different classrooms and classes in schools and nurseries or research laboratories where natural ventilation is unfeasible.

    Portable air purifiers purify the air through various filters. They generally contain a pre-filter for larger impurities, an active carbon filter that cleans the air of volatile organic compounds, and a HEPA filter that serves as a barrier to 99,97% of all particles and allergens in the air, including viruses, bacteria, mold spores and pollenso that adequate artificial ventilation can be achieved.

    Previous studies already demonstrated that the presence of the SARS-CoV-2 virus could be detected and monitored in large spaces thanks to the analysis of environmental samples collected using sponges. Now, a multidisciplinary team of researchers from different institutions, including the Castilla-La Mancha Health Service (SESCAM), the Care Center for People with Serious Intellectual Disabilities (CADIG), the Combustion Research Institute and Air Pollution (ICCA) and the Research Group in Health and Biotechnology (SaBio) of the Instituto de Investigación en Recursos Cinegéticos (IREC – CSIC, UCLM, JCCM), have developed and validated a method for detecting the SARS-CoV-2 virus in safe environments, where only asymptomatic people were found, by analyzing the HEPA filters that are part of air purifiers, with the aim of monitoring its presence and detect possible new outbreaks in indoor spaces where the virus spreads more easily.

     

    Marta Sánchez-Sánchez and Alberto Moraga-Fernández, researchers from the IREC SaBio Group, processing the sponges and HEPA filters obtained during the study campaign (Source: La Tribuna newspaper of Ciudad Real).

    For this work, a monitoring campaign was carried out for 13 weeks in 3 different indoor settings, in which the analysis of the HEPA filters was carried out weekly as a surveillance method to detect the presence of genetic material (RNA) of the SARS-CoV-2 virus by PCR. Among the study scenarios, all in the province of Ciudad Real, included a class of a primary school (8 and 9 years), two classrooms of a nursery for children (2 to 3 years) and a socio-sanitary residence inhabited by Adults. The HEPA filters of the air purifiers located in these rooms were replaced weekly, and were subsequently analyzed in order to detect the presence of SARS-CoV-2 viral RNA. Along with these filters, environmental samples were also collected from different common contact surfaces, such as tables, chairs and door handles, for subsequent analysis of viral detection.

    For the validation of the study method, before starting the monitoring of these three scenarios, Filters from spaces similar to those included in the study and in which the presence of a person infected with SARS-CoV-2 had previously been confirmed were analyzed.. After confirmation of the case, the filters were collected 6 days after the start of the quarantine of that space and its closure. The studies for the validation of the method were carried out both in school classrooms and in the social and health residence for adults. As proof of a negative control, the analysis of filters that had not been exposed to indoor air was carried out in the same way, being in all cases negative in the PCR. The analyzes of the filters located in spaces where there had been the presence of an infected person gave positive results in PCR in all cases, thus confirming and validating the study method.

     

    Graphic description of the method developed to assess the presence of the SARS-CoV-2 virus in indoor settings: in the air using HEPA filter testing and on surfaces using sponge testing.

    During the monitoring campaign, no symptomatic case was detected. However, the results obtained in the sampling of the first week showed a positive HEPA filter (with the presence of RNA of the SARS-CoV-2 virus) in the socio-sanitary residence, which led to carrying out individual diagnostic tests on the residents and all the personnel who had access to the interior where the air purifying equipment whose HEPA filter was installed was found to be positive, confirming an asymptomatic positive case of one of the workers.

    It should be noted that the presence of this asymptomatic infected person would not have been confirmed at the initial moment of infection if it had not been for the performance of this study, with the possible consequences of contagion to the rest of the users who shared that stay in the residence. During the rest of the monitoring campaign, no filter or sponge was positive for the presence of the virus, nor was any positive case detected in the people who normally occupied the sampled locations. It's important pointing that the study began after the third wave, when the incidence of the disease was very low, not being able to start earlier essentially due to the delay in obtaining the necessary authorizations to carry out the study.

    HEPA filter analysis can predict the presence of asymptomatic cases and thus control possible outbreaks of SARS-CoV-2.

    These results suggest that environmental surveillance through the analysis of HEPA filters could help to detect and control outbreaks of the virus in indoor spaces, reducing the economic costs of individual analysis and speeding up the diagnostic process.

    The project has been developed thanks to the public and private collaboration of the Provincial Council of Ciudad Real, the Provincial Delegations of Education, Health and Social Welfare, the Microbiology Service of the General University Hospital of Ciudad Real, the Ethics Committee for Drug Research , the Integrated Care Management of Ciudad Real, the Poblete City Council, the Eurocaja Rural Foundation and the company Quantum Spain, together with the collaboration of the study participants (children and residents, their tutors, teachers and workers at the residence of study).

    The scientific publication of this research is available at: