When trying to control the spread of viral pathogens it is crucial to consider all means suitable for breaking the potential chain of infection. In this regard, the virucidal performance of domestic laundry processes might act as one important parameter to reduce virus transmission ^[1][2][1,2], although surfaces have been shown only to play a minor role in the transmission of respiratory viruses, such as SARS-CoV-2 ^[3][4][3,4]. The COVID-19 pandemic has certainly created a growing demand for an effective hygiene framework to deal with viral pathogens in everyday life settings. Although common domestic hygiene standards should always be applied in a targeted way [5], e.g., hand hygiene, respiratory hygiene and general home hygiene, hard surfaces and textiles can be regarded as fomites and thus need to be addressed to lower the infection risk by viral pathogens including enteric viruses (norovirus and rotavirus), respiratory viruses (influenza and coronavirus) and other viruses, e.g., herpes simplex virus and poliovirus [6]. The main building blocks of the viral structure are the nucleic acid core and the surrounding protein capsid for non-enveloped viruses and an additional lipid bilayer for enveloped viruses, harboring surface proteins or glycoproteins which mediate the connection to specific receptor sites on susceptible host cell surfaces. The structure of surface proteins is determined by the viral nucleic acid, while the lipid bilayer is derived from the host cell membrane.

2. Factors Influencing the Antiviral Efficacy of Laundering

Like the cleaning performance, the antimicrobial effect of laundering follows the principle introduced by Herbert Sinner (1960) according to which a washing process is determined by four variables: temperature, mechanical action, chemistry and time ^[7][31].

2.1. Temperature

Temperature directly affects the inactivation of microbial cells and viral particles on reduction on laundry items by thermal impact; it also helps in physico-chemical removal of organic matter and plays a role in the activation of peracids. Studies suggest that temperatures of 60 °C might be able to completely reduce the vast majority range of microorganisms on textiles, including non-enveloped viruses, even without the use of activated oxygen bleach ^{[8][9][10][11][12][13][14][15][16][17][18][19]}[11,32,33,34,35,36,37,38,39,40,41,42]. For decades, a trend towards lower temperatures has been observed, which comprises the major means to save energy during the washing process and to promote clothes longevity; however, this also has been shown to decrease the antimicrobial performance of laundering; moreover, it has been shown that particularly enveloped viruses are readily inactivated even when lower temperatures and a bleach-free detergent is used [2]. In North America laundering conditions typically are much lower than European with the cold wash being on average 16 °C compared 30 °C for Europe with minimal impact on overall health. Thus, laundering at lower temperatures can considered to deliver a sufficient hygiene level, unless non-enveloped viruses are concerned.

2.2. Chemistry

Even without the use of antimicrobial agents, the detergency effect will lead to a considerable decrease of the microbial cells on a fabric surface, which can be attributed to the mechanical removal rather than a killing effect; however, adding commonly used biocidal substances such as bleaching agents or quaternary ammonium compounds might increase the antimicrobial efficacy ^{[2][20][21][22][23][24][25][26]}[2,43,44,45,46,47,48,49]. Gerba et al. reported the effect of washing and drying in a home washing machine on enteric viruses (Adenovirus, Rotavirus, and Hepatitis A virus) ^[25][48]. The enteric viruses remained infectious throughout laundering without bleach and transmission from contaminated to uncontaminated swatches occurred, while a great impact on 99.99% virus reduction by bleach was observed ^[25][48]; furthermore coronaviruses are effected by bleach (concentration 0.1%), either Ethanol (62–71%) showed to be quite effective (>4 log) ^[27][10]. Especially SARS-CoV-1 und SARS-CoV-2 are described as unusually stable beside the envelope, even though they are rather lipophilic and sensitive to solvents and surfactants ^[28][21]; moreover, a wide pH stability could be shown for SARS-CoV-2 ^[28][21]. The virucidal efficiency of current laundering processes for enveloped viruses is due to the virion sensitivity to detergents and aided by bleach and they can be inactivated even at 20–30 °C ^[2][29][2,50]. Virus particles, enveloped or not are able to survive washing at 30 °C without detergent and can contribute for transmission, so that recommendations for the inactivation, use detergents or elevated temperatures ^{[2][8][28][29]}[2,11,21,50]. Traditionally, in many parts of the world, chlorine bleach has been used for this purpose, whereas in other countries activated oxygen bleach (AOB) can be predominately found. The active ingredient percarbonate releases hydrogen peroxide in aqueous solutions at higher temperatures. Using bleach activators such as TAED (tetraacetylethylenediamine) or NOBS (sodium nonanoyloxybenzenesulfonate) can push this reaction below 60 °C and can thus significantly increase the antimicrobial efficacy even at lower temperatures ^[11][15][18][34,38,41]. The use of quaternary ammonium compounds (QAC) like benzalkonium chloride (BAC) and dimethyl didecyl ammonium chloride (DDAC) is common for hygiene rinse aids since anionic surfactants which are widely used in laundry detergents are not compatible with those. QACs can interact with the surface of negatively charged textiles, and thus providing a persisting antimicrobial effect. The exerted antimicrobial efficacy highly depends on the type of microorganism and residual detergent left in the rinse. While the use of QACs results in a high reduction of gram-positive bacteria even at low concentrations, fungi or Pseudomonads are much less affected by these biocides ^[30][51]. There is no comprehensive data on the antiviral efficacy of QACs in laundering, although these substances are active against certain viruses as well ^[31][52]. Chin et al. described the viral reduction below levels of detection, when 0.10% of benzalkonium chloride is used against SARS-CoV-2 ^[32][19].

2.3. Time

As proposed in the Sinner’s principle, it has been shown that a decrease in temperature can be compensated by increasing other variables, in particular the wash cycle time ^[11][34]; however longer washing cycles cannot completely restore the antimicrobial performance of laundering for certain microorganisms at very low temperatures ^[11][34]; it must be noted that this has been proven using bacterial and fungal test strains, but has not applied to virucidal effects so far. Nevertheless, it can be assumed that at least a limited compensation of lower temperatures by longer times will also be seen for virucidal effects. Honisch et al. described the interplay between time and temperature in current washing machines that aim to decrease the washing temperatures in order to save energy and in turn exhibit very long programme durations ^[11][33][34,53].

2.4. Mechanical Action

Finally, the construction type of the washing machine (i.e., horizontal vs. vertical axis) can be regarded a factor influencing the antimicrobial efficacy by mechanical action. Although evidence is still poor, it was shown by Honisch et al., that the mechanical action of the washing machine might help to physically remove microbial cells from textiles ^[21][44]. At least for enveloped viruses, carrying a cell membrane as the outer layer, these principles may apply as well.

3. Studies on the Antiviral Efficacy of Laundering

In terms of their role in the transmission of infectious diseases, textiles and laundering have not been studied intensively so far. Bloomfield et al. (2011) compiled available studies on the potential infection risk associated with contaminated textiles, also regarding viruses, resuming that textiles are considered potential fomites, also for viral diseases ^[25][34][15,48]; however, due to the low frequency of association it may not be a large source of transmission. Some studies described the microbial community associated with laundry as a resulting consortia from skin-associated bacteria, microorganisms from the environment and from washing machine biofilms ^{[35][36][37][38]}[54,55,56,57]. As mentioned before, laundering can be used to disinfect contaminated textiles; however, when addressing a laundry-related risk of virus infections, not only the reduction by the laundering process itself has to be considered. Likewise, the specific interaction between virus and textile and the viral survival vary significantly depending on the virus species and the way of handling textiles in the laundry process ^[33][53]. Parameters like ambient organic matter or the ability to associate with surfaces via connection by the spike glycoproteins as anchors, stabilize the virion ^[26][49]. Enteric viruses are far more resistant to laundering procedures and antimicrobial agents. In general, non-enveloped viruses are more difficult to inactivate, whereas enveloped viruses are easier to inactivate in a washing process, because the phospholipid envelope can be disrupted by the detergent. Some non-enveloped enteric viruses have been proven to survive longer on textiles, and Poliovirus survives at room temperature on cotton up to 84 days ^[39][58]. Higher temperatures (30–40 °C) decreases the duration of persistence of coronaviruses, i.e., MERS-CoV, Alphacoronavirus 1 (TGEV), MHV, while lower temperature (4 °C) increase the persistence of MHV and Alphacoronavirus 1 (TGEV) up to 28 days ^[27][10]. HCoV 229E seems to be stable at room temperature and 50% relative humidity (RH) ^[27][10], whereas temperature at 6 °C seems to have a greater impact on enhanced survival rates than RH ^[40][59]. In general the influence of humidity on persistence has been described inconsistently ^[41][60]. Scientific studies on laundry hygiene are mostly focused on bacteria and fungi, whereas viral pathogens have been considered much less ^[33][42][53,61]; however, there are some studies dealing with viruses on textiles in particular and the antiviral efficacy of laundry-associated processes. Apart from a direct transmission to humans, a transfer of viruses from contaminated textiles to other textiles in a common household laundry process has been described as well ^{[2][26][29][43]}[2,49,50,77], presumably during laundering; however, this phenomenon might also take place on dry textile and might especially concern non-enveloped viruses. For instance, Herpes simplex virus (HSV) seems to be more stable in low humidity and at low temperatures ^[44][96], so cross-contamination may even occur outside the washing machine, e.g., when handling insufficiently decontaminated laundry. In contrast to that, a complete inactivation of HSV in common household laundry was observed when using a detergent containing activated oxygen bleach ^[29][41][50,60]. Heinzel et al. confirmed a good virucidal efficacy even at 40 °C by conventional household laundry detergents (0.4%) for enveloped viruses like Bovine Viral Diarrhea Virus (BVDV), Vaccinavirus (VACV) as well as for non-enveloped viruses, such as Bovine Parvovirus (BPV), Poliovirus and Simianvirus (SV40); it was shown that the virus particles were not only completely removed from the textiles, but chemically inactivated, leading to a reduction of >5 log [2]. Belonging to the most difficult viruses to inactivate, Noroviruses (NoV) have been shown to be resistant against a great range of chemical agents, thus detergents containing activated oxygen bleach and washing temperatures above 50 °C are required for a complete inactivation when taking the washing machine only into consideration ^[8][45][11,97]. Rotaviruses are inactivated by a common disinfectant ingredients including 70% ethanol, 6% hydrogen peroxide, chlorine, povidone—iodine, hypochlorite (without faecal matter), ultraviolet radiation and heat, not all of them being applicable in laundry. Professional and domestic laundry is mostly followed by a drying cycle at high temperatures (e.g., in a tumble dryer), this means can be considered as well, when estimating the antiviral efficacy of a common laundry process. In some studies, processing steps using heat (80 °C) or even high pressure have also been investigated ^[46][98]. Tests on the stability of SARS-CoV on surfaces (polystyrene) showed that SARS-Coronavirus remains infectious for up to six days, in particular in presence of protein load; however, at 56 °C a quick and complete inactivation was observed ^[47][16], suggesting the efficacy of laundering and/or drying at high temperatures is sufficient. After a potential transmission of SARS-CoV-2 from contaminated dry surfaces has been discussed ^[48][18], MacIntyre et.al. investigated the efficacy of laundering on medical face masks and non-medical face masks (two- layered cotton mask) in a randomised trial ^[49][99]. The masks were reused and cleaned on a daily base, either by hand-wash with soap, tap water and air-dried, or by hospital laundry. The study showed that non-medical face masks can be as protective as medical masks, if washed as recommended by WHO (≥60 °C, with detergent) ^[49][50][99,100].