University of São Paulo: Greenhouse with dry heat is more effective to decontaminate surgical masks, shows study conducted at HC


Research carried out at the Hospital das Clínicas (HC) of the Medical School of USP (FMUSP) during the pandemic evaluated surgical masks and respirators (mask N95) from five manufacturers, in addition to the equipment used in four decontamination methods: dry heat (oven), autoclaving (autoclave), thermal disinfection (thermal drying machine) and hydrogen peroxide (steam machine). Photo: Provided by the researchers
OUse of a dry heat oven is the most effective decontamination method for inactivating the covid-19 virus in surgical masks and respirators (N95 masks) used by healthcare professionals. The result was obtained in a research carried out at the Hospital das Clínicas (HC) of the Medical School of USP (FMUSP) during the pandemic. The greenhouse, in addition to maintaining the filtration capacity of protective equipment, allows them to be safely reused.
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The conclusions of the work are in an article published in December in the International Journal of Infectious Diseases . “In the first phase of the study, carried out in the laboratory, different methods of decontamination of masks were used and parameters such as permeability and filtering of the masks were evaluated”, report researcher Marina Farrel Côrtes and professor Silvia Figueiredo Costa, from the Instituto de Medicina Tropical ( IMT) and FMUSP, who were part of the group that carried out the work. “In the ‘real life’ phase, performed at the HC, the presence of the SARS-CoV-2 virus was assessed using the RT-PCR test before and after decontamination.”


Surgical mask subjected to greenhouse decontamination cycles; despite the gradual decrease in resistance, they showed no practical differences after five cycles; filtration capacity remained above 92% after four dry heat decontamination cycles – Photo: Provided by the researchers
The research evaluated surgical masks and respirators (mask N95) from five manufacturers, in addition to the equipment used in four decontamination methods: dry heat (oven), autoclaving (autoclave), thermal disinfection (thermal drying machine) and hydrogen peroxide (washing machine). steam). “Fragments of the positive masks by RT-PCR were subjected to virus cultivation in order to assess whether the virus remained viable and could be transmitted”, point out the researchers.



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In addition to visual inspection to identify signs of deterioration, elasticity of the elastic and nasal clip, the masks were evaluated for air permeability (air passage under pressure), burst resistance (rupture) and breathability. “In this respect, the difference in pressure was verified for the air to pass from the inside to the outside. The smaller this pressure difference, the less resistance the material offers for the passage of air, that is, the more breathable the material is ”, affirm Marina and Silvia. “The efficiency of particle filtration and DNA retention capacity was also measured, that is, how much the mask can prevent the passage of DNA molecules.”


Respirator after decontamination cycles; none of the respirators subjected to viral culture showed positivity and particle filtration capacity remained above 96% after two decontamination cycles in the greenhouse and with hydrogen peroxide vapor – Photo: Provided by the researchers
Among the 14 masks used by health professionals during shifts, SARS-CoV-2 RNA was detected by RT-PCR in two of them before decontamination, in particular those collected in the first week of the pandemic, when there were no face shields available and before adopting security measures. “Only one mask remained positive by RT-PCR after decontamination in the greenhouse, which does not mean that the virus was viable, only that the genetic material remained there”, highlight the researchers. “None of the respirators subjected to viral culture were positive.”

Decontamination

As all decontamination techniques performed well after one cycle, the effect of up to five cycles with an oven and autoclave, the most common methods in hospitals, was studied. “The third decontamination cycle in the autoclave significantly affected the mechanical resistance of surgical masks and detached the nasal clip from respirators, so it would not be a suitable method for more than two cycles”, observe Marina and Silvia. “On the other hand, decontamination in the greenhouse, despite the gradual decrease in burst resistance and pressure differential in surgical masks, did not show any practical differences after five cycles.”

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Surgical masks lost their ability to filter DNA after the tenth decontamination cycle in the greenhouse, while respirators maintained their retention capacity. “The number of decontamination cycles did not affect the particle filtration efficiency of the masks”, say the researchers. “The particle filtration efficiency indicates that surgical masks remained with filtration capacity above 92% after the four decontamination cycles tested in the greenhouse and the respirators above 96% after at least two decontamination cycles in the greenhouse and with steam of hydrogen peroxide. ”

In addition, 33 health professionals completed a questionnaire considering the seal and breathing performance of the masks before and after decontamination. “The study found that the mere use of the mask, that is, factors such as dressing and removing, moisture in the breath and speech, already changes its performance in the tests analyzed when compared to the new mask”, explain the researchers. “However, this difference is low, less than the differences found between the different brands of masks and, therefore, without practical significance.”


Silvia Figueiredo Costa and Marina Farrel Côrtes: use of the mask, that is, factors such as dressing and removing, moisture in the breath and speech, changes performance in the tests when compared with the new mask, however this difference is low, less than the differences found between the various brands, without practical significance – Photo: Personal archive
The research is reported in the article “Descontamination and reuse of surgical masks and respirators during COVID-19 pandemic”, published on December 22 in the International Journal of Infectious Diseases . The work had the participation of Marina Farrel Côrtes, Evelyn Patricia Sanchez Espinoza, Saidy Liceth Vásconez Noguera, Anna S. Levin, Silvia Figueiredo Costa, from the Medical Research Laboratory (LIM- 49-Bacteriology Laboratory) and Lucy Santos Villas Boas, Noely Evangelista Ferreira, Tania Regina Tozetto-Mendoza, Maria Cassia Mendes Correa, from the Medical Research Laboratory (LIM 52-Virology Laboratory), from IMT and from the FMUSP Department of Infectious and Parasitic Diseases.

At the HC, the Surgical Center (Aline Alves Silva, Marion Elke Sielfeld Araya de Medeiros) and the Hospital Infection Control Commission, Hospital Infection Control Group and the Pandemic Crisis Committee of Covid-19 (Thais Guimaraes, Ana Rubia Guedes, Leila Suemi Harima Letaif, Amanda Cardoso Montal). Also participating were researchers Fernando Gonçalves Morais, from the Physics Institute (IF) and Vanderley M. John, from USP’s Polytechnic School (Poli), as well as Rayana Santiago de Queiroz, from the São Paulo State Institute of Technological Research (IPT) and Adriana Coracini Tonacio de Proenca, from Hospital São Camilo, in São Paulo.

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