The U.S. has officially entered the post-pandemic era. Part of the changes that came with it is the new guidance the Centers for Disease Control (CDC) announced for buildings and indoor establishments that aim to prevent COVID-19 transmissions.

In its new guidance posted on the agency's website Friday, the CDC advised indoor settings to have "at least 5 air changes per hour of clean air in occupied spaces."

According to the CDC, since airborne viral particles easily spread between people indoors than outdoors, protective indoor ventilation practices should be followed to reduce the airborne viral concentrations and the overall exposure of occupants.

The CDC encouraged building owners and operators to seek expert consultation, so they can come up with or follow mitigation strategies that would work efficiently for the type of indoor setting they have.

"It's a monumental shift. We haven't had this. We haven't had health-based ventilation standards," Joseph Allen, the director of the Harvard Healthy Buildings Program, told CNN of the new guidance.

Per the new guidance, potential COVID-19 transmissions can be prevented indoors when proper ventilation mitigation strategies that help reduce viral particle concentration are applied. Such strategies are designed to reduce the risk of contracting the virus.

The CDC clarified that not all interventions work in all scenarios. Moreover, they do not eliminate the risk completely. What they do is reduce the spread of the disease among occupants.

The guidance includes provisions for other indoor settings aside from buildings and establishments, including public transportation vehicles such as buses, trains, school buses and rideshares.

"I am pleasantly surprised to see CDC add this guidance. I do find it ironic that they finally published ways to end the pandemic at the same time as declaring it is over," University of California at San Diego and the Scripps Institution of Oceanography atmospheric chemist Kimberly Prather reacted to the guidance.

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"If they had broadcast and implemented these changes at the beginning, there never would have been a pandemic," she additionally told CNN.

The guidance came after the World Health Organization announced last week that the COVID-19 pandemic was ending based on the latest data on hospitalizations and deaths due to the virus.