How much carbon dioxide (CO2) is in the air you are breathing right now? You might want to know.

Higher-than-recommended levels of CO2 — the stuff we breathe out — suggests inadequate air exchange, a common condition in poorly ventilated rooms. The air can be more stagnant than fresh, and contain higher levels of contaminants like dust, dander, mold and chemicals. It could also mean a higher concentration of the aerosolized liquids that carry COVID-19.

This is why evidence of elevated CO2 levels in poorly ventilated classrooms is a big deal. Public school students have returned to school in full force. Many of them study in close quarters in air-conditioned rooms with the windows closed.

At the same time, masks are optional and the general public increasingly behaves as if the pandemic is behind us.

It is not. Many people are still contracting and dying from COVID-19, and the most contagious version yet, BA.5, is ascendant.

So the state Department of Education (DOE) and others are right to be concerned about elevated levels of CO2 recently reported in seven Oahu schools. In 73 rooms (63 of them classrooms), the CO2 concentrations ranged from 1,500 parts per million to more than 2,000. Between 400 and 1,000 is considered normal for a classroom. The average for outdoors — the safest place to be when COVID-19 is about — is 400.

Ironically, a major culprit could be air conditioning in classrooms, a top priority of the Ige administration to keep students more comfortable and better able to learn. The DOE’s guidance on ventilation includes turning off the A/C in rooms with a lot of windows that can be opened, and using box fans to circulate air.

Otherwise, the DOE is distributing HEPA air purifiers, which don’t reduce CO2 levels but catch some of the airborne gunk floating about. Schools also can go DIY, building a cube-shaped air cleaner out of a box fan and several high-grade filters.

These are imperfect solutions, and the DOE will need to maintain an extensive program of CO2 monitoring, reporting the results publicly. It’s also worth noting that the number of classrooms affected appears to be relatively low; there are 12,000 public school classrooms, and testing implicated about 1,200 of them.

But students and their parents shouldn’t count on the odds, or assume that school maintenance will keep them safe. The virus is stubbornly contagious; the best defense continues to be wearing a mask indoors and being vigilant about safety, especially if exposed.