The ¾ rule: recommendations for voluntary outdoor masking


By Patricia B. Mirasol, Reporter 

INDIVIDUALS who are fully vaccinated against coronavirus disease 2019 (COVID-19), physically distanced, and outdoors may not need to wear their masks, according to an epidemiologist.  

Remember the ¾ rule of voluntary masking when outdoors, said Dr. John Wong, an epidemiologist and chair of the Department of Health drug price advisory council, at a Sept. 23 session organized by the Healthcare Professionals Alliance Against COVID-19 and 702 DZAS FEBC radio.  

The ¾ rule entails asking four questions:   

Am I fully vaccinated?  
Am I physically distanced?  
Am I outdoors?  
Do I need to mask?  

“To keep safe, you need any three out of the four conditions,” he said.   

“Vaccination is the easiest to comply with. Once you’ve done it, you can forget about it. All the other behaviors you have to pay attention to,” Dr. Wong said.  

Physical distancing means keeping a personal bubble of 1–2 meters. Masks, meanwhile, have to be high quality, with N95, KN95, and KF94 variants being Dr. Wong’s recommendations.   

Masking has other benefits too, he added, citing fewer deaths due to tuberculosis and pneumonia.  

“The only change in behavior was masking … Even if you’re outdoors, you’re not only protecting yourself from COVID-19 but also from other respiratory diseases,” he said. “The ¾ rule is a good rule of thumb for everyone.”  

WHAT ABOUT INDOORS, IN CARS?The calculus changes when the setting moves indoors, even if a venue’s occupants get a negative result in their antigen test, which is not as sensitive as an RT-PCR (real-time reverse transcription-polymerase chain reaction) test. 

An indoor space’s carbon dioxide level can be measured with carbon dioxide sensors, Dr. Wong said. If the level is less than the maximum accepted concentration of 1,000 ppm (parts per million or µg/m3), then distancing and masking are encouraged.  

“If you can distance, you may not have to wear a mask. If you can’t, better to wear [one],” he said. 

“We don’t have too many regulations about air quality. … We go to malls, we don’t know what the air quality is,” Dr. Wong added. “I think this needs to be enforced better.”  

 Ventilation is also key when riding in a car with passengers of unknown vaccination status. “To improve ventilation, request that two of the windows are opened at least three inches,” he said. 

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