The Centers for Disease Control and Prevention on Friday released a host of new guidance on reopening schools, even as an increasing number of school districts across the country have either already opted for online-only instruction or have been forced to shut down amid coronavirus cases.

The updates now include far more lengthy and detailed recommendations on some topics that the agency had previously only provided a few sentences on. Most notably, the CDC added a bulleted list of recommendation on the issue of ventilation, which has recently become a particular flashpoint in New York City's plan to reopen its schools, many of which have outdated or poor HVAC systems. Although experts say it is possible to contract coronavirus by touching infected surfaces, there is increasing evidence that the virus is mostly spread by respiratory droplets.

Schools are now being advised to use of a host of equipment to improve air flow, including fans, portable air filtration systems, and an ultraviolet light that could knock out the virus in rooms with little to no air circulation. The latter method is estimated by the CDC as costing $1,500.

The agency also outlined more guidelines for school cafeterias, a critical nutritional resource in a country where more than 30 million children receive free lunch. Although an earlier version advised against having students eat in cafeterias, the new update offers strategies to accommodate such communal settings by having children remain at least 6 feet apart while waiting on line and eating at tables, as well as disinfection of tables and chairs between each use. Other suggestions included installing touchless payment methods, another costly upgrade that many schools facing huge budget shortfalls cannot afford.

In another shift, the CDC also no longer recommends that schools perform screenings of all students in grades K-12. According to the last guidance, which was updated in May, the CDC had written, "If feasible, conduct daily health checks (e.g., temperature screening and/or or symptom checking) of staff and students."

It was not clear why the CDC reversed its guidance on health screenings. The agency has also not recommended universal testing of teachers and students.

In the area of preparation, the agency is now advising schools to develop an "emergency operations plan" with regard to the daily monitoring of coronavirus data as well as responding to infections.

But the document still leaves important questions unanswered, namely what number of infections in the community or school should prompt a full closure. The agency has in the past maintained that one confirmed case is not a sufficient reason to close a school. Nonetheless it has not specified how many confirmed cases should prompt a school closure.

During a phone news conference on Friday, Dr. Robert Redfield, the director of the CDC, dodged questions on the level of community transmission or cases that would should force schools to reconsider reopening. Instead, he continued to emphasize the importance of reopening schools, which President Donald Trump has repeatedly demanded in tweets and other public remarks.

But while he maintained that schools were "a safe environment for our kids," he cautioned, "They are not islands by themselves. They are connected to the communities that surround them.”

He also acknowledged that school officials needed to earn the confidence of both teachers and parents.

At the start of the briefing, Redfield held up a newly published study by the CDC on Rhode Island childcare centers which found limited secondary transmission, where someone infected in the daycare passes along the virus to an outside person.

Out of nearly 19,000 children enrolled in the state's childcare programs between June 1st and July 31st, Rhode Island health officials identified only 52 confirmed and probable cases.

According to Redfield, the study showed that child care centers can reopen "when things are done with vigilance."

To date, evidence both in and outside the U.S. have suggested that infections are inevitable once schools reopen. A suburban school district in Atlanta recently asked nearly 1,200 students and staff members to quarantine shortly after the school year began. On Friday, the Associated Press reported that at least 41 schools in Berlin had reported infections within two weeks of schools reopening. Germany, which has a positivity rate of 2.77%, is being closely watched by other European countries as they seek to open their classrooms.

With more than 1 million students, New York City is attempting to become the only major school district in the country to reopen schools with some in-person instruction. But Mayor Bill de Blasio has faced intensifying pressure from both teachers and principals to delay the start date. On Wednesday, the teachers' union issued a list of safety demands and threatened to strike if they were not met.

Governor Andrew Cuomo has said that positivity rate across the state, which has been below 1% for 14 straight days, is low enough for schools to reopen. Nonetheless, he has stopped short of endorsing New York City's plan. Appearing on The Today Show this morning, he was asked hypothetically whether he would elect to send his child to a New York City public school.

"I would have a lot of questions. Parents do have a lot of questions," Cuomo replied, adding, "This is a risky proposition no matter how you do it."

On Monday, the Los Angeles Unified School District, the second largest school district in the country, announced a plan to test nearly 700,000 students and 75,000 employees before resuming in-person classes. The proposal represents the most ambitious school testing program to date, with collaborations with the University of California, Johns Hopkins University, Stanford University, Microsoft and the insurers Anthem Blue Cross and Health Net. It has received the backing of the teachers union.

Appearing on WNYC's The Brian Lehrer Show on Friday, de Blasio dismissed LA's testing initiative.

"From what we're seeing, that was a very big announcement without a lot of specifics behind it," he said. "There does not appear to be an actual, tangible plan in Los Angeles to actually test the kids and the staff in real time. And they're working with an all remote system. That sounds like a very vague, if not noble vision there."

New York City plans to ask teachers and staff to get tested a week before school starts and then at least once every month. But the testing will not be required.

De Blasio said that his administration was currently in talks with the union about the amount of testing the city could provide.