Many U.S. schools have poor air circulation, which will exacerbate the coronavirus threat when some of them reopen this month. Aging air conditioning and heating systems allow classroom air to become stagnant, meaning the virus will linger in the air around the students and teachers instead of being blown into air ducts and out of the school. At one New York City school, teachers are threatening not to return until their central air conditioning system is fixed. It last worked in 2013. Some schools will be teaching classes outdoors, even in colder climates. They are buying large tents and heaters to protect students from the weather.