It is a widely held belief that "negative ions" in the atmosphere are good for you (see this article on WebMD).
Some sources (see this word document from Fayetteville State University) say that positive ions have negative effects.
How good is the evidence that positive ions are bad?
There isn't any strong evidence that positive ions are bad for your health.
Psychological effects of air ions have been reported for more than 80 years in the media and scientific literature. This study summarizes a qualitative literature review and quantitative meta-analysis, where applicable, that examines the potential effects of exposure to negative and positive air ions on psychological measures of mood and emotional state. ... No consistent influence of positive or negative air ionization on anxiety, mood, relaxation, sleep, and personal comfort measures was observed. Negative air ionization was associated with lower depression scores particularly at the highest exposure level. Future research is needed to evaluate the biological plausibility of this association.
Source: Perez V, Alexander DD, Bailey WH. Air ions and mood outcomes: a review and meta-analysis. BMC Psychiatry. 2013 Jan 15;13:29. doi: 10.1186/1471-244X-13-29.
