Montgomery County residents should wear masks outdoors starting Monday, the Health Department recommended, a step that reflects the unabating concern and uncertainty about the transmission of the delta variant of the coronavirus.

People should wear masks outdoors whenever they cannot stay at least six feet apart from others, county officials said in a news release, citing a level of virus transmission that has fallen into the “high” category for the last two weeks.

The recommendation applies to all, regardless of vaccination status, while the county remains in high transmission. Every county in Pennsylvania — and most of the country — is in the high category.

Drexel epidemiologist Michael LeVasseur said the masking recommendation makes sense, given the infectiousness of the delta variant.

“Masking adds an additional layer of protection to prevent infection,” he said in an email. “As cases increase in our region, we should be considering how to reduce our exposure to infection, and this certainly includes the measures we recommended in the era prior to widespread vaccine rollout.”

Last month, Oregon become the first state to adopt a mask mandate outdoors even for vaccinated individuals in a bid to prevent hospitals from coming under too much stress.

Montgomery County’s recommendation is stricter than that of the CDC, and the first county in the region to make a renewed call for masking outdoors since the spring, when the vaccine became available and outdoor masking regulations began to ease.

The CDC said in its August guideline update that masks were generally not needed in outdoor settings but recommended considering a mask in crowded outdoor settings or close-contact activities in areas with high transmission.

Philadelphia requires masks outdoors at all non-seated events with more than 1,000 attendees. Montgomery’s new general outdoor mask recommendation excludes anyone engaging in sports activities. The county also recommends masks indoors.

Masking is currently recommended indoors by the CDC.