Table 1 Study characteristics

Silverman and Kornblueh 1957[27]^a,b

Evaluate effect of negative and positive ions on the human electroencephalogram and sleep

Crossover experiment

Not reported

10 healthy adults and 2 additional subjects with chronic stationary neurologic conditions

12

McGurk, 1959[17]^c

Evaluate effect of negative and positive ions on self-reported feelings of comfort

Crossover experiment

Single-blind (subjects)

10 college-aged males

10

Yaglou, 1961[19]^b

Evaluate effect of negative and positive ions on relaxation

Crossover experiment

Single-blind (subjects)

25 healthy adults (age range: 22–51)

25

Yaglou, 1961[19]^b

Evaluate effect of negative and positive ions on relaxation and sleepiness

Crossover experiment

Single-blind (subjects)

6 arthritic patients (age range: 30–62)

6

Assael et al., 1974[11]^b

Evaluate effect of negative ions on human electroencephalogram

Crossover experiment

Double-blind

10 healthy participants (age range: 20–65) and 10 subjects receiving tranquilizers

20

Albrechtsen et al., 1978[37]^b,c

Evaluate effect of negative and positive ions on human well-being and mental performance

Crossover experiment

Single-blind (subjects)

Study 1: six women (age range: 20–30) chosen at random; study 2: 12 subjects (age range: 19–45) selected because they appeared to be most sensitive to ionization

Study 1: 6 Study 2: 12

Charry and Hawkinshire, 1981[15]^a

Evaluate effect of negative and positive ions on mood

Crossover experiment

Single-blind (subjects)

85 adults (age range: 18–60; mean age: 30)

85

Hawkins, 1981[38]^{b, c}

Evaluate effect of negative and positive ions on subjective well-being and comfort

Crossover experiment

Double-blind

Study groups based on three areas of variable air ionization levels within the building (area 1: 20 women; area 2: 32 adults; and area 3: 54 adults)

Area 1: 20

Area 2: 22

Area 3: 54

Tom et al., 1981[34]^a,b

Evaluate effect of negative ions on human performance and mood

Randomized controlled trial

Double-blind

56 adults (age range: 17–61; mean age: 23)

56

Buckalew and Rizzuto, 1982[12]^a,b

Evaluate effect of negative ions on subjective feelings of mood and psychological state

Randomized controlled trial

Double-blind

Two groups of 12 paid male volunteers matched on age, education, physical condition, and smoking habits (age range: 20–30; mean age: 22.8)

24

Dantzler et al., 1983[25]^a

Evaluate effect of positive and negative ions on somatic symptoms and mood changes

Crossover experiment

Double-blind

9 patients with bronchial asthma (age range: 35–64)

9

Baron et al., 1985[28]^a

Evaluate effect of negative ions on self-reported affect/mood

Crossover experiment

Single-blind (subjects)

71 male undergraduate students

71

Deleanu and Stamatiu, 1985[29]^a,b,d

Evaluate effect of negative ions on psychiatric symptoms

Experimental (no control group)

Not reported

112 patients with neurasthenias, psychoses, or personality disorders

112

Gianinni et al., 1986[16]^a

Evaluate effect of negative and positive ions on anxiety, excitement, and suspicion

Crossover experiment

Double-blind

14 university-affiliated volunteers

14

Gianinni et al., 1986/1987[30]^a

Evaluate effect of positive ions on anxiety and excitement

Crossover experiment

Double-blind

12 adult male volunteers

12

Finnegan et al., 1987[40]^c

Evaluate effect of negative ions on personal comfort rating

Crossover experiment

Single-blind (subjects)

26 adults working within 5 different rooms of an office building

26

Hedge and Collis, 1987[18]^a

Evaluate effect of negative ions on mood

Crossover experiment

Double-blind

28 healthy women (age range: 19–58)

28

Lips et al., 1987[13]^b,c

Evaluate effect of negative ions on well-being and comfort

Crossover experiment

Double-blind

18 normal, healthy employees working in one of two rooms, whereby room 1 had windows providing air ventilation and room 2 was mechanically ventilated

18

Misiaszek et al., 1987[14]^a,b

Evaluate effect of negative ions on manic behavior and sleep

Experimental (phase I: no control group; phase II: with-in subjects, repeated measures)

Phase I: No Blinding Phase II: Double-blind

8 manic patients (age range: 22–49)

Phase I: 4 Phase II: 4

Reilly and Stevenson, 1993[33]^a

Evaluate effect of negative ions on anxiety

Crossover experiment

Single-blind (subjects)

8 healthy men (age range: 19–25)

8

Terman and Terman, 1995[6]^d

Evaluate effect of negative ions on seasonal depression

Randomized controlled trial

Double-blind

25 patients (mean age: 38.2 ± 11) with winter depression

Low-density negative ion group: 13 High-density negative ion group: 12

Watanabe et al., 1997[35]^a,c

Evaluate effect of negative ions on mood and pleasantness

Crossover experiment

Single-blind (subjects)

13 healthy adults (age range: 21–49; mean age: 26.4)

13

Terman et al., 1998[8]^b,d

Evaluate effect of negative ions on sleep and seasonal depression

Crossover experiment

Double-blind

124 subjects (age range: 18–59; mean age: 39.4 ± 9.8) with seasonal affective disorder

124 (20 randomized to high-density negative ionization and 19 randomized to low-density negative ionization)

Nakane et al., 2002[10]^a

Evaluate effect of negative ions on anxiety

Crossover experiment

Not reported

12 female undergraduates (age range: 18–22)

12

Iwama et al., 2004[39]^b

Evaluate effect of negative ions on tension

Randomized controlled trial

Double-blind

44 patients randomized to the control and 51 patients randomized to receive treatment (mean age among men: 37 ± 18; mean age among women: 43 ± 20)

95

Goel et al., 2005[22]^b,d

Evaluate effect of negative ions on sleep and chronic depression

Randomized controlled trial

Double-blind

32 patients (age range: 22–65; mean age: 43.7 ± 12.4) with non-seasonal chronic depression

32 (22 randomized to low- or high-density)

Goel and Etwaroo, 2006[5]^a,b,d

Evaluate effect of negative ions on depression, total mood disturbance, and anger

Randomized controlled trial

Single-blind (subjects)

118 mildly depressed and non-depressed college students (mean age: 19.4 ± 1.7)

118 (59 randomized to low or high density)

Terman and Terman, 2006[7]^b,d

Evaluate effect of negative ions on sleep and seasonal depression

Randomized controlled trial

Double-blind

99 adults with seasonal depression (94 cases) and bipolar II disorder (five cases) (age range: 19–63; mean age: 40.4 ± 10.4)

99 (39 randomized to low or high density)

Gianinni et al., 2007[26]^a

Evaluate effect of negative ions on manic states

Crossover experiment

Double-blind

24 manic male patients (age range: 23–29; mean age: 26.7)

24 (20 analyzed)

Malcolm et al., 2009[32]^a,b

Evaluate effect of negative ions on positive affective memory

Randomized controlled trial

Single-blind (subjects)

30 healthy subjects (age range: 18–28) randomized to either receive high-density negative air ion exposure or to a control condition

30

Flory et al., 2010[4]^d

Evaluate effect of negative ions on seasonal depression

Randomized controlled trial

Single-blind (subjects)

73 university-affiliated women (age range: 18–51; mean age: 20.8 ± 5.69) with seasonal affective disorder

73 (38 randomized to low or high density)

Malik et al., 2010[9]^a

Evaluate effect of negative ions on psychological stress

Crossover experiment

Single-blind (subjects)

20 regular users of computers as part of their job (age range: 24–35; mean age: 28.9)

20

Dauphinais et al., 2012[24]^d

Evaluate the effect of negative air ions on seasonal depression

Randomized controlled trial

Double-blind

44 adult patients (20 in the low-density group) with bipolar depression

20

Harmer et al., 2012[31]^a,b,d

Evaluate the effect of high-density negative air ions on emotional processing in patients with seasonal depression

Randomized controlled trial

Double-blind

21 adult patients with seasonal depression; 21 controls. Mean ages of groups between 30–35 years

42