Room air cleaners offer portability and provide filtration in homes without forced air HVAC systems. They are offered in a variety of sizes, and the filtration types generally fall into two categories: HEPA filters and electronic electrostatic devices.
In a year-long, randomized, parallel-group study, Francis et al.  measured the clinical outcomes for the use of HEPA air cleaners in both the bedroom and living room of 30 adult asthmatics who were both sensitized to and lived with (against medical advice) an indoor cat or dog. The control group and the active group also used cyclonic HEPA vacuums twice per week. Primary end points in combined asthma outcomes (bronchial reactivity and treatment requirements) were statistically improved in the treatment group over the controls. Secondary end points of lung function and allergen levels improved in both groups and were not statistically significant.
Gore et al.  measured the amount of cat allergen found on nasal personal air samplers using HEPA PRACs in five homes with indoor cats over 4 separate days. They found significant reductions in the amount of measured Fel d 1 when the PRAC was used with the cat in the room (P < 0.01) but no difference 3 h after the cat was removed from the room. The study was limited by the small number of homes studied, the short duration of use of the PRACs, and the fact that the mean ventilation rate of the PRACs used was only 4.09 m3/h (range, 8.9–2.8 m3/h). It did demonstrate that even small allergens such as cat will fall out into settled dust within a few hours and will not be affected by air filtration. Because settled dust is stirred and becomes airborne during the times people and pets are active in the household, this study does support air filtration as a means of reducing exposures.
An excellent overall discussion of air filtration is provided by Batterman et al.  in reporting the results of a 2-month study of the effects of continuous use of HEPA PRACs in the homes of cigarette smokers. The PRAC decreased PM concentrations by 30% to 70%, depending on size fraction and occupant activities. They concluded that PRACs can reduce exposure of PM that has been associated with the exacerbation of asthma and other respiratory symptoms, but that further research is necessary to determine whether such filters improve respiratory health.
Bernstein et al.  reported the first study to investigate the effect of dehumidification in conjunction with HEPA filtration as an intervention to control airborne culturable mold levels. They were able to show reduction in airborne mold levels, but it was not possible in this study to determine whether the reduction of airborne mold spore levels was due mainly to dehumidification or air filtration intervention.
A recently published study by Sulser et al.  of children sensitized to cat or dog compared sham versus HEPA PRACs placed in the living room and bedroom. The research, conducted in 1999 and 2000, excluded children also sensitized to dust mite or known mold exposure. A significant reduction in nocturnal symptoms was observed, as was a trend in the active group toward an improvement in bronchial hyperresponsiveness.
A standard test bedroom was devised by Hacker and Sparrow  for the evaluation of six commercially available air cleaners. The brands of the units were not identified, but photos of the units were provided. Three were electrostatic precipitators, and three used HEPA filtration. Two of the HEPAs were PRACs, and one a stationary model designed for use at the headboard end of a bed to provide filtration to the “sleep breathing zone” (SBZ). One of the electrostatic units was a tower unit with no fan. Particulate counts were taken at 1-min intervals over an 8-h period at the approximate position of the SBZ. To measure device performance, they reported three components of the appliances effectiveness: 1) the time-wise variation of particulate concentration measured in the experimental breathing zone of a sleeping person, 2) average particle size concentration over the 8-h test period, and 3) efficiency of particle removal as measured over the inflow/outflow of the units. Reduction of particle concentration ranged from nearly complete removal of all particles by the SBZ unit to no benefit over natural particle-settling rates by the tower unit with no fan. (This unit appears to be a well-known national brand highly promoted by infomercials that was also found ineffective in the studies by Macintosh et al. [25••].) The electrostatic devices did not perform as well as a group. There was no assessment of the amount of ozone these machines may generate. All three HEPA appliances had excellent efficiency measures. The SBZ unit provided the best overall performance because of its special design to clean the breathing zone.
The concept of focusing on creating a particle-free SBZ dates back to the previously mentioned study by Zwemer and Karibo , which looked at a filtration system built into a headboard of a bed. Three studies, one by Morris et al.  and more recent ones by Pedroletti et al.  and Stillerman et al. [36•], looked at various HEPA SBZ filtration systems. Morris et al.  studied the use of a laminar flow HEPA unit that could be positioned over the SBZ. They studied 14 ragweed-sensitive individuals with allergic rhinitis during the ragweed season. The duration of filter use was 3 weeks. Outcome measures included recording morning and evening symptom scores, the Juniper Rhinitis Quality of Life Questionnaire, and the Epworth Sleepiness Scale, along with tolerability rating. The total group showed an average of 26% reduction in morning symptoms (P = 0.025 compared with baseline) and an average of 24% reduction in evening symptoms (P = 0.013).
Pedroletti et al.  studied 22 patients 12 to 33 years of age with mild to moderate asthma in a 10-week, double-blind, placebo-controlled, crossover trial. All were sensitized to cat or dog, and 76.2% were also sensitized to dust mite, birch tree pollen, or both. The device tested provided laminar airflow of cooled, HEPA-filtered air directed to the SBZ. It is designed so that a “shower” of cooled, filtered air displaces air containing particles, including allergens, in the SBZ. The primary effectiveness measure was the change in quality of life between active versus placebo treatment, assessed by the mini-Asthma Quality of Life Questionnaire. In addition, changes in exhaled nitrous oxide and spirometry were assessed. At the end of the 10 weeks, the treatment group showed improvement in both the mini-Asthma Quality of Life Questionnaire (P < 0.05) and exhaled nitrous oxide (P < 0.05). Both effects occurred within the first 2 and 5 weeks, respectively, of treatment. There were no significant changes in pulmonary functions from baseline.
Stillerman et al. [36•] studied a combination of a HEPA filtration attached to a dust mite–proof pillow encasement. This unique configuration also was designed to provide a particle-free SBZ. They studied 35 patients with perennial allergic rhinoconjunctivitis (75% moderate to severe) in a randomized, double-blind, placebo-controlled, crossover trial lasting 42 weeks. Patients were assessed by skin prick testing. A total of 89% were dust mite sensitive, 57% cat sensitive, and 47% were sensitized to dog. Allergen levels from vacuum dust sampling in the bedroom and bedding were also measured. Only 43% percent of the patients had significant exposures to at least one of the sensitizing allergens. (This would imply that symptoms might be due to other allergens not tested [eg, mold], or from a nonallergic trigger, including particulates.) Allergy medications were prohibited. Instantaneous and reflective Total Nasal Symptom Scores (TNSS) were recorded daily. Quality-of-life scores were recorded weekly using the Nocturnal Rhinoconjunctivitis Quality of Life Questionnaire. Particle counts showed that active and placebo devices reduced SBZ particles measuring at least 0.3 μm from a median background level of 712,680/cubic foot to median levels of 80/ and 548,050/cubic foot, respectively. The active treatment group had significant improvement versus placebo (P < 0.001) in both instantaneous on waking and overnight reflective TNSS. Daytime TNSS did not improve. Nocturnal Rhinoconjunctivitis Quality of Life Questionnaire scores were also significantly better in the active treatment group, especially in patients with moderate to severe symptoms.