A Cochrane review included 6 studies with a total of 106 subjects. No results were combined as the studies were all of a crossover design. Effects of negative ion generators (5 studies): No study reported a significant difference in lung function between ionised and control air (morning PEF, 3 studies; FEV1, 1 study). There were no significant differences in symptoms or beta-2 agonist usage between ionised and control air in 3 studies. Effects of positive ion generators (1 study): This study demonstrated that although positively ionised air was associated with a larger fall in FEV1 with exercise, this did not reach statistical significance. Baseline FEV1 was not demonstrated to be significantly different between treatment groups.
Comment: The quality of evidence is downgraded by study quality (unclear allocation concealment) and by imprecise results (limited study size for each comparison).