Viruses are the biggest problem for air filters. Smaller viruses, which are usually smaller than 300 nm, are not completely eliminated by HEPA filters. Consistent with this claim, the nanotextile monolayer did indeed contain coronavirus 229E. It has been shown previously that HEPA filters do not capture viruses adequately. This may be due to several factors. Firstly, viruses can enter the air filter through defects in the material itself, caused for example by pleating of the individual layers of the filter medium. Pinhole leaks may be another cause of filtration capacity failure (Harstad and Filler 2007). Other studies on HEPA filter efficiency also report limited capture of viral particles by HEPA filters (Helmbuch et al. 2007). HEPA filters operate at the level of a HEPA filter that filters 99.97% of MMPs. The results of the present study are quite consistent with our previous studies revealing poor virus capture in HEPA filters. In particular, coronavirus 229E penetrated the HEPA filter in our previous study of a home air purifier equipped by a HEPA filter (Obitková and Pavlík 2019). Adenovirus is also significantly small, with a diameter in the range of 70 to 100 nm. This suggests that it can penetrate through the filter. The occurrence of this virus on the inlet surface of the filter may be due to the droplet mode of transmission of this virus (Baron 1996). Droplets are removed from filtered air by HEPA filter more efficiently than for example aerosols. The COVID-19 pandemic prompted more rapid development of personal protection measures, including the development of face masks and respirators made of nanomaterials. Nanotextiles are a promising means of air filtration. Our selected monolayer of nanotextiles captured coronavirus 229E. This pathogenic virus is small, suggesting that nanotextiles could be a sufficient means of air filtration. The nanotextile used in this experiment has several key properties. The 50 nm pore size covers the diameter of most human viral pathogens. If the nanotextile served only as a sieve, no viruses of our interest would penetrate the chosen monolayer of nanotextile. On the other hand, the small pores of the nanotextile may also be an obvious disadvantage. Nanotextiles could not be used as the sole filter medium due to the clogging of the pores by dust or similar larger particles present in the filtered air. Air filters made of nanomaterials designed as a nanofibrous monolayer with a microfibrous support can have a significantly higher efficiency in eliminating submicron aerosols (Podgórski et al. 2006). From this perspective, our chosen nanotextile can significantly increase the filtration efficiency of a standard HEPA filter.

As mentioned above, larger particles are usually a challenge for nanomaterial air filters due to surface loading. The filtered particles from the circulated air are trapped only on the surface of the nanofiber filter. They do not penetrate deep into the filter medium as in the case of conventional air filters. In our last experiment, we encountered surface loading of the nanotextile monolayer after 100 h of filtration (Obitková and Pavlík 2019). Moreover, some viruses have short lifespan in the external environment — adenoviruses can survive on the fabric for less than 24 h, and influenza A and B viruses can survive on selected surfaces for 24–48 h (Pirtle and Beran 1991). Therefore, we decided to replace the nanotextile monolayer three times with a replacement interval of approximately 33 h. From a technical point of view, we are unable to estimate the lifetime of the nanomaterial because no information was provided by the manufacturer of the nanomaterial used in the experiment.

The Respiratory SARS CoV-2 Panel real-time PCR detection kit running on the QIAStat DX® 1.0 platform was a suitable choice to cover most of the respiratory viruses we were primarily looking for. The Respiratory Virus Panel is designed to cover all major causative agents of upper and lower respiratory tract infections. The operation of the analyzer is simple and the results obtained were sufficient for our purpose. The RT PCR technique provides very reliable detection of viral nucleic acid. For future research, the quantification of viral load on the nanotextile monolayer or the viability of the detected viruses could be investigated. Although the minimum infectious dose for virus-induced diseases is very low, we believe that quantification of HEPA filter-transmitted viruses could at least be of interest. Furthermore, the QIAStat SARS CoV-2 panel used did not allow differentiation between different types of adenoviruses. To find a specific type, we would use an adenovirus-specific analysis kit in any type of RT PCR cycler.

The HEPA filter was contaminated with adenovirus. In the spring season, when respiratory infections are more common, a second member of the experimental household may have been the source of the adenovirus. The chosen multiplex RT PCR analyzer does not allow the specification of adenovirus, so we cannot accurately determine the type of adenovirus. We can only assume that the detected adenovirus originated from the respiratory tract, as adenoviral conjunctivitis is more typical for the summer season and adenoviral infections of the digestive or urinary tract are spread through urine or stool (Goering et al. 2018). On the other hand, the source of coronavirus 229 E is well known and was found in the nasopharynx of one of the members of the experimental household. This household member may have been in the incubation period of the upper respiratory tract infection.

The current experimental setup was designed to be easy to install with a small number of samples. In comparison to other studies of portable air purifiers equipped with HEPA filters provided indoors — home or school, (Rodríguez et al. 2021; Myers et al. 2022), our experiment is unique in that swabs of the HEPA filter surface and the nanotextile monolayer demonstrate the presence of viral contamination. In their study, Rodriguez et.al. conducted a study on the effectiveness of portable air purifiers in eliminating SARS-CoV-2 in several households in different cities and demonstrated an 80% elimination efficiency of air purifiers. Lindsay et al. also simulated the effectiveness of an air purifier in a single room (Lindsley et al. 2021). We know that the experiment from one household could not be able to show objective information. Contamination of the HEPA filter or the monolayer of nanotextiles could have occurred during the replacement of the nanotextile or during laboratory procedures on both the HEPA filter and the nanomaterial. Despite the simple experimental setup and small sample size, we have provided valuable data in investigating the ability of nanomaterials to trap viruses and increase the efficiency of conventional air filters.