Changing from gas to climate-friendly electric appliances often involves expensive retrofits. A growing list of companies offer stoves, heat pumps and water heaters that make it easier and cheaper.

Boosting ventilation and filtration not only lowers the risk of transmission, it's also linked to higher test scores and better health. But making upgrades is more complex than it sounds.

Changes to ventilation — everything from opening windows to making pricey upgrades to HVAC systems — can help reduce the risk of the coronavirus being spread inside a building.