Headaches, sleepiness, low attention spans and a general sense of feeling unwell are just some of the side effects of spending too long indoors. Scientists say it's all about ions, or, more specifically, negative ions. Ions are charged particles in the air; some are charged negatively, and some are charged positively. Negative ions, put in non-scientific terms, are basically oxygen ions with an extra electron attached, produced through water molecules, which is why they are in such beautifully abundant supply near fresh, flowing water supplies such as rivers, streams, seas and waterfalls. This also explains how refreshing it can be to have a have a shower, swim or take a bath when we are feeling fatigued. But these negative ions, in plentiful supply in natural settings, are dangerously low in the average air-conditioned office, over-heated home or stuffy car. The air around waterfalls, for example, can contain anywhere from 30,000 to 100,000 negative ions per cubic centimetre. Compare this number to a stuffy car or office, which can contain zero to, at most, a few hundred per cubic centimetre.