Some air isn’t healthy because it doesn’t have enough oxygen. If you leave a gas or oil heater on in a closed room, the lack of oxygen will eventually lead to carbon monoxide poisoning, which is very dangerous.

But, then, what is healthy air? Healthy air has plenty of oxygen, isn’t dusty or gritty, and doesn’t have any gases that are bad for our bodies. Have you ever felt refreshed after spending time in the countryside or in a forest where the air is clean? Air that isn’t dirty can feel “delicious”, even though it doesn’t have any taste or smell.