To the Editor—Kalu et al ^{Reference Kalu, Henderson, Weber and Haessler1} propose that universal masking in routine patient-care interactions be made a permanent component of standard precautions. This topic is important to consider given the experiences over the past few years of the COVID-19 pandemic, but it warrants fulsome consideration of the evidence and potential harms/costs prior to widespread implementation.

There is a long history in medicine of implementing interventions based on expert opinion and/or observational data, with subsequent randomized controlled trial (RCT) data showing that the intervention is not effective or even harmful. ^{Reference Prasad and Cifu2} We note that the studies cited by the authors as evidence in support of universal masking are laboratory-based simulations and observational studies. ^{Reference Kalu, Henderson, Weber and Haessler1} However, laboratory studies do not take into account human factors and adherence with interventions under real-world conditions. Observational studies are subject to numerous biases and cannot prove that the intervention caused the observed outcome. Indeed, a recent systematic review of RCT evidence of masks for preventing transmission of respiratory viruses was unable to draw firm conclusions as to their effectiveness. ^{Reference Jefferson, Dooley and Ferroni3} In particular, the 2 included RCTs of universal masking among healthcare workers (HCWs) prior to the COVID-19 pandemic showed no statistically significant decrease in influenza-like illness, with wide confidence intervals. ^{Reference Jacobs, Ohde and Takahashi4,Reference MacIntyre, Seale and Dung5} The systematic review did not find any RCTs of universal masking in HCWs during the pandemic, but the lack of clear benefit for prevention of COVID-19 in community-based studies suggests that this is an a topic worthy of further study.

Even if universal masking was proven to be effective at preventing transmission, the benefit of any intervention has to be weighed against the costs and the risks. Clearly there are financial costs to purchasing masks, and a formal cost–benefit analysis would be worthwhile to determine the cost of preventing each transmission event. In addition, it is important to consider the environmental cost of disposing of large numbers of masks, including the addition of many tons of waste into landfills and the release of heavy metals and volatile organic compounds as the masks degrade. ^{Reference Li, Sathishkumar and Selahuddeen6} Although the goal is to have no transmission of infections within healthcare settings, resources are limited and consideration of whether there may be a point of diminishing returns is warranted. For example, how much additional benefit is there with masking all the time versus with symptomatic patients only, or with patients at high risk for severe infection?

Furthermore, there are potential adverse events related to universal masking. Although this has not been consistently measured, up to 75% of participants in mask RCTs report adverse events. ^{Reference Jefferson, Dooley and Ferroni3} Even among HCWs, 40.4% reported adverse events in one of the studies, most commonly discomfort and breathing difficulties. ^{Reference MacIntyre, Seale and Dung5} Also, potential harms to patients that are not often assessed in studies and may be difficult to quantify can significantly affect patient care. For example, patients who have difficulties with hearing or language may face barriers in communicating with masked HCWs. Finally, universal mask policies generally result in extended use of masks, which may paradoxically increase the risk of healthcare-associated infections as HCWs wear more heavily contaminated masks in their interactions with patients. ^{Reference Chughtai, Stelzer-Braid and Rawlinson7} This was the traditional reason for changing masks between patients, a habit that has been lost without study during the pandemic.

Although the COVID-19 pandemic has raised awareness at a societal level of the need to ensure HCW and patient safety, we urge our colleagues in infection prevention and control and public health to apply the principles of evidence-based medicine, to carefully study the impacts, and to quantify the real-world benefits prior to recommending the implementation of universal masking as a permanent part of standard precautions.