WHAT IS CONSIDERED AN "EFFECTIVE" AIR CLEANING DEVICE?

Transcription

1 WHAT IS CONSIDERED AN "EFFECTIVE" AIR CLEANING DEVICE? R Shaughnessy 1 and R Sextro 2 1 Indoor Air Program, University of Tulsa, 600 S College Ave, Tulsa, OK 74114, USA 2 Lawrence Berkeley National Laboratory,One Cyclotron Road, Berkeley, CA ABSTRACT The most widely used method in the United States to assess the performance of new portable air cleaners is the procedure described in the American National Standards Institute (ANSI)/Association of Home Appliance Manufacturers (AHAM) AC The test protocol is straightforward, yielding a performance metric that is based on the measured decay rate of contaminant concentrations with the air cleaner operating compared with the measured decay rate with the air cleaner turned off. The resulting metric, the clean air delivery rate (CADR), permits both an intercomparison of performance among various air cleaners and a comparison of air cleaner operation to other contaminant removal processes. In this paper we comment on the testing process, discuss it s applicability to various contaminants and evaluate the resulting performance metrics for effective air cleaning. INDEX TERMS air cleaner, effectiveness, clean air delivery rate, air cleaning device, portable air cleaner, particles INTRODUCTION Portable air cleaners are aggressively marketed to a public that is eager to buy into the broad spectrum of claims being hyped. The increased use of these devices is accompanied by if not influenced by - a fundamental belief by consumers that the air cleaners are providing an improved indoor air environment. However there is a wide variation in the performance of air cleaners that is dependent on the specific air cleaner design and various indoor factors. About the same time that air cleaner sales began increasing in the 1970 s and 80 s, it was recognized that there were no uniform testing methods that could be applied to give consumers a means of making performance-based choices. Lawrence, et al. (1981) report on the potential benefits of portable air purifiers, but note operational and evaluation variables that require consistency to discern the effectiveness of the units. Whitby, et al. (1983) make reference to the lack of standard methods for testing/rating of air cleaning devices, and expressed concern regarding the sweeping and general claims being used by manufacturers of the devices. Consumer Reports (1961) reported over 40 years ago on the need for validation of these claimed results, and the lack of published results on the effectiveness of the air cleaners. DISCUSSION Applicable Performance Standards Two early papers described essentially the same evaluation methodology and reported on the performance of some of the then-available consumer portable air cleaners (Whitby, et.al., 1983; Offermann, et al., 1985). Whitby.reports on a dynamic method for measuring smoke, hydrocarbon gas removal, and odorant emission rate, developed and applied to the evaluation of six different air purifiers. Offermann discusses a similar method used to evaluate eleven air cleaning devices for control of environmental tobacco smoke particles. The methodology described in these two papers formed the basis for the first published ANSI/AHAM standard AC-1(AHAM, 1989) and for subsequent air cleaner performance evaluations published by AHAM and by Consumers Union ( the most recent published by Consumer Reports 2003). The AHAM term Clean Air Delivery Rate (CADR) was created, in lieu of the previous terms employed by Whitby and Offermann, to describe the equivalent of clean air provided to the space by an air cleaner. In addition, other researchers have evaluated and utilized this performance metric for other types of air cleaners and found the approach to be valid and useful in estimating the effects of the devices in various room sizes, or in comparing air cleaning to ventilation as an indoor quality control technique (see, for example, Daisey and Hodgson, 1989; Nelson et. al., 1993; Shaughnessy, et al., 1994; Brown et al., 1999; Foarde, et. al., 1999; Nazaroff, W., 2000). The performance metric used to evaluate the performance of air cleaning devices is based on a fundamental mass balance of contaminants within a space. Concentrations of contaminants in indoor air are dynamic and result from the competition between various source and removal processes. Mathematically, these processes can be 2970

2 described by: dc dt = S / V + Pλ C ΛC. (1) v o i The first two terms on the right hand side of equation 1 describe the source terms. The first is the volumetric source term, describing any indoor contaminant emission source, where S is the emission rate in mass per unit time and V is the volume of the indoor space. The second is infiltration of contaminants from outdoors, where λ v is the air infiltration rate (in units of inverse time), P is the penetration factor and C o is the outdoor contaminant concentration. The last term in equation 1 describes the indoor removal processes, where Λ represents all first order removal processes (in units of inverse time) and C i is the indoor contaminant concentration. The removal processes can be broken down into their constituent parts, for example Λ = λ v + λ d + λ ac (2) where λ d is the removal rate due to deposition onto surfaces and λ ac is the removal rate due to air cleaner operation. When performing air cleaner testing in a room-size chamber, sufficient concentrations of the test contaminant are introduced into the chamber so that concentrations due to any infiltrating contaminant can essentially be neglected. The actual testing commences after the contaminant source is turned off, so S=0. Thus, during testing equation (1) reduces to dc dt = ΛC i = (λ v +λ d +λ ac )C i. (3) The solution to equation (3) is C(t) = C(0)exp( Λt) = C(0)exp[ (λ v + λ d + λ ac )t]. (4) It is useful to rearrange the terms in equation (4) and take the natural log of both sides, ( ) Λ=λ v +λ d +λ ac = lnc(0)/c(t). (5) t In practice, C(0) is the contaminant concentration at the start of the analysis - usually chosen to be at the time when conditions in the test chamber are well mixed, and t is the elapsed time between the starting concentration, C(0), and the ending concentration, C(t). In order to explicitly account for room size, the ANSI/AHAM standard (AHAM AC ) defines CADR as CADR = V ( Λ Λ ) = V ( λ + λ + λ λ λ ) = Vλ. (6) AC noac v d ac v d As can be seen in equation (6), CADR is based on the measured difference in decay or removal rates with the air cleaner in operation and with no air cleaner operation. The units of CADR are flow, i.e., volume per unit time. The resulting metric, (CADR), permits both an intercomparison of performance among various air cleaners and a comparison of air cleaner operation to other contaminant removal processes. Note also that fans are often used to provide mechanically driven air movement through an air cleaner; but that neither the definition of an air cleaner nor the AHAM developed AC-1 (AHAM, 2002) testing protocol requires that a fan be used. Judging the Effectiveness of Portable Air Cleaning Systems To be effective, the air cleaner removal rate must compete with other removal processes that occur within the space such as surface deposition (for particles) or sorption (for gases), indoor air reactions (typically for gases), and ventilation (outdoor air exchange). ac The concept of air cleaner effectiveness has been used to examine acceptable values for CADR. Effectiveness, ε, 2971

3 has been defined by Nazaroff 2000) as the difference in indoor concentration due to air cleaning (C noac - C AC,) compared to the "no-cleaning" case, C noac : ε= C noac C ac C noac. (7) Using the derivation for CADR and a representation for C noac and C ac, we obtain a relationship between effectiveness and the contaminant removal processes ε= CADR V(λ v +λ d ) + CADR = CADR /[V(λ v + λ d )] 1+ CADR/[V(λ v +λ d )]. (8) The degree of air cleaner effectiveness to have a meaningful effect on contaminant concentrations in the indoor space has been the subject of discussion for many years. The closer the effectiveness is to 1, the more ideal the performance of the air cleaner is in removal of the contaminant. Figure 1 shows air cleaner effectiveness as a function of the ratio between CADR and the other removal processes (ventilation and (for particles) deposition), CADR/ [V(λ v + λ d ]. AHAM has a generally accepted performance recommendation (effectiveness) of 80% reduction, in steady-state particle concentrations, related to air cleaner operation. The AHAM performance recommendation (effectiveness) of 80% shown as a dotted line in Figure 1 ( AHAM Recommended ), requires that the ratio of CADR/ [V(λ v + λ d ] be a factor of 4 (i.e.., particle removal rate by air cleaning is four times larger than removal by ventilation and deposition combined). As an illustration, the combination of ventilation rate and particle deposition rate is ~1.05 h -1 (representative of typical ventilation rate of 1 h -1 and deposition rate for small particles of 0.05 h -1 ), thus for a room size of 15 x 15 x 8 ft (= 1800 ft 3 or 51 m 3 ), the combined removal rate based on ventilation and deposition is 30 ft 3 /min (50 m 3 /h), leading to a minimum CADR of 120 ft 3 /min (200 m 3 /h). Whitby (1983) in his early development of the dynamic model for evaluating air cleaners stated that for a device to be effective in air cleaning, the unit must provide a minimum of one air change per hour (of cleaned air) to the room intended for use. Whitby s recommendation would equate to a 50% reduction in contaminant. Based on Whitby s recommendation for a minimum effectiveness of 50% the air cleaning removal rate must be equal to that due to ventilation and deposition combined (shown as dashed line Whitby 1983 ). The Swedish Asthma and Allergy Association (SAAA, 1999) supports the more accepted AHAM guidance that an 80% reduction in contaminant concentration is necessary to produce meaningful reductions in contaminant concentrations indoors. The guidance is mainly based on particulate removal, however the SAAA clearly states that the same guidelines are applicable for gas adsorption based air cleaners. Note, we have chosen to discuss CADR in the units used by AHAM (ft 3 /min) and have provided the metric equivalents, as appropriate. 1 effectiveness 0.9 (AHAM Recommended) (Whitby, 1983) Removal Ratio = CADR/[V(λ v +λ d )] Figure 1. Effectiveness of airborne contaminant removal as a function of the ratio between air cleaner performance and the other removal processes, ventilation and deposition (for particles) (adapted from Nazaroff, et al., 2000). 2972

4 In practice, many air cleaning devices on the market today have rated CADR values, with respect to particulate removal, significantly higher than the minimum suggested by Whitby for an occupied space. Figure 2 provides a graphic representation of CADR values necessary to meet the AHAM recommended 80% effectiveness value and Whitby s suggested 50% value CADR (ft3/min) CADR-80 (AHAM) CADR-50 (Whitby) Area (sq feet) Figure 2. Clean Air Delivery Rate as a function of indoor area for two effectiveness values, 80% as recommended by AHAM and 50%, based on Whitby, et. al. (1983); graph applies to removal of smaller particles, less than ~1 micron in size. Application of Portable Air Cleaning Systems, Specific to Particles AHAM currently makes use of the CADR as a means of rating the particle removal performance of portable air cleaners on the market. Three particle types have been selected for these evaluations: environmental tobacco smoke - hereinafter "ETS" (particle size range, µm diameter), dust (particle size range, µm diameter), and paper mulberry pollen (particle size range, 5-11 µm diameter). Although, the AHAM CADR ratings for air cleaners (AHAM updates the air cleaner performance data on a regular basis (AHAM, 2004)) show only a minor dependence on particle size, the effectiveness of particle removal afforded by the air cleaner is very much dependent upon particle size, as described by equation (8). The change in deposition rate as particle size increases, significantly impacts the overall effectiveness of the air cleaner itself. Deposition loss rates vary over almost three orders of magnitude as particle sizes range from ~0.2 to over 10 µm (Thatcher, et.al. (2002); Xu, et.al. (1994), Fisk, et.al. (2002)). An examination of three particle size regions representing three different types of aerosols of interest reveals a significant variance in associated deposition rates. For small-sized particles less than 0.8 micron, e.g. ETS, the deposition removal rate is ~0.05 h -1. For medium-sized particles ranging from 1-4 microns, e.g. house dust, some allergens such as cat allergen, the deposition removal rate is ~1 h -1. For large-sized particles ranging from 6-20 microns, e.g. pollen and allergens, such as dust mite and cockroach feces, the deposition removal rates is ~10 h -1. When compared with typical residential ventilation rates (~ 1 per h) (Sherman and Matson, 1997), particle deposition losses for small particles have little impact on the particle removal ratio described in equation (8). On the other hand, losses due to deposition of medium size particles are about the same as those due to ventilation. Finally, large particles deposit at rates that are much higher than the equivalent loss rate due to ventilation. These differences in particle deposition losses have an important effect on the resulting CADR needed to meet the effectiveness criterion. Using equation (8) and a ceiling height of 8 feet (2.4 m) we recast the equation in terms of floor area, A, a more commonly used descriptor of room size than is room volume, to yield CADR = A 8ε ( λ v + λ d ) 1 ε (9) Using this equation and the ventilation and particle deposition parameter values described above, we develop the relationship between room size, CADR and particle size category, as shown in Figure 3. Three lines display 2973

5 CADR as a function of indoor space area calculated for small, medium and large particle categories, based on an air cleaner effectiveness of 80%. For small particles, a room area of 400 ft 2 (37 m 2 ) requires an air cleaner with a CADR value of 225 ft 3 /min (382 m 3 /h). For particles in the medium size category, an air cleaner with a CADR of 425 ft 3 /min (722 m 3 /h) is needed for the same size room. Note that only one of the air cleaners listed by AHAM has a CADR for dust above 425 ft 3 /min. For large particles, the CADR value required to achieve an 80% reduction in particles for a 400 ft 2 room is over 2300 ft 3 /min (3910 m 3 /h), which is well beyond any portable air cleaner tested by AHAM (the largest CADR listed for pollen is 400 ft 3 /min (680 m 3 /h ). CADR (ft 3 /min) CADR-80 (small) CADR-80 (med) CADR-80 (large) CADR-67 (large) CADR-50 (large) Area (ft 2 ) Figure 3. Clean Air Delivery Rate as a function of indoor space area and particle size. Volume is calculated based on a ceiling height of 8 ft (2.4 m). CADR-80, -67, and -50 are, respectively, the CADR values based on 80 %, 67% and 50% reduction in particle concentrations due to air cleaner operation. The particle size categories are those described in the text. SUMMARY In summary, an indepth analysis indicates that many of the particle air cleaners tested by AHAM (2004) meet the 80% effectiveness criterion for small (e.g., ETS) particles when applied to the appropriate room size (see equation (9) and Figure 3). On the other hand, meeting this criterion for larger particle sizes (e.g. particles with diameters 2 µm and greater) is more difficult, even though the AHAM testing procedure includes both dust and pollen as test aerosols. In the case of filter-based air cleaners, filtration efficiency should improve in most cases as the particle size increases from sub-micron to super-micron particle diameters (see, for example, Fisk, et. al. (2002). However, particle deposition losses also increase rapidly with increasing particle size, thus increasing the CADR needed to maintain 80% removal based on air cleaner performance alone. The relationship among the three key elements affecting air cleaner efficacy room size, CADR and particle size category was presented in Figure 3 for specific air cleaner effectiveness categories. Overall, these results underscore the need to consider which particle size category is of greatest interest. For homes where ETS particles are the main concern, air cleaning effectiveness of 80% or greater can be achieved with moderate to large CADR ratings. On the other hand, for pollen control, only the combination of small room size and large CADR ratings will yield particle removal effectiveness of 80% or greater. For typical room sizes greater than 200 ft 2 (18 m 2) removal effectiveness of 80% (for large-sized particles e.g. pollen, dust mite and cockroach allergen) cannot be feasibly achieved by air cleaners currently on the market due to the very high CADRs required to meet this performance goal. This may account for the finding, stated in the Clearing the Air Report (National Academy of Sciences, 2000), that there is limited evidence indicating that particle air cleaning is associated with a reduction in the exacerbation of asthma symptoms. The Report goes on to state that theoretical and empirical data suggest that air cleaners are most likely to be effective in reducing indoor concentrations of particles smaller than approximately 2 microns. However, much of the airborne allergen appears to be within larger particles. This again, is consistent with the statements formulated above. 2974

6 Finally, it is important to remember that much of this discussion has used effectiveness as an important consideration. Recall (see equation 7) that this is a relative measure of air cleaner efficacy. In some situations, an 80% reduction (for example) in particle concentrations may not be sufficient to achieve an absolute concentration (and hence exposure) goal or requirement. Source control is and will always be the preferred approach to reduction of contaminants within an indoor setting, while air cleaning is representative of a supplemental means of managing airborne pollutants. REFERENCES AHAM 1989, AHAM (AC-1), Association of Home Appliance Manufacturers Washington, D.C. AHAM 2002, ANSI/AHAM AC-1, Association of Home Appliance Manufacturers, Washington, D.C. AHAM 2004, Directory of Certified Room Air Cleaners, Edition 3, Association of Home Appliance Manufacturers, Washington, DC, (July) Brown SK. 1999, Assessing the Performance of Room Air Cleaners Using a Room Environmental Chamber, Proceedings of the 7th International Conference on Indoor Air Quality and Climate, Edinburgh, Scotland. Consumer Reports, 1961, Report on Room Air Purifiers, Consumer Reports, April, pp Consumer Reports, 2003, Air Cleaners: Behind the Hype, Consumer Reports, October, pp Daisey JM. and Hodgson AT. 1989, Initial Efficiencies of Air Cleaners for the Removal of NO2 and Volatile Organic Compounds, Atmospheric Environment, Vol. 23, No.9, pp Fisk WJ., Faulkner D., Palonen J. and Seppanen O. 2002, Performance and Costs of Particle Air Filtration Technologies, Indoor Air 12: Foarde KK., Myers EA. and Hanley JT. 1999, Methodology to Perform CADR Type Determinations with Microbiological Aerosols, Aer. Sci. and Tech., Vol. 30, pp Lawrence JC., Lilly HA. and Wilkins MD. 1981, Evaluation of a Portable Air Purifier, J.Hyg. Camb., 86:203. National Academy of Science 2000, Clearing the Air: Asthma and Indoor Air Exposures, Institute of Medicine, National Academy Press, Washington, DC. Nazaroff WM. 2000, Effectiveness of Air Cleaning Technologies, Proceedings of Healthy Buildings 2000 Conference, Helsinki, Finland, Volume2, pp Nelson PR., Sears SB. and Heavner DL. 1993, Application of Methods for Evaluating Air Cleaner Performance, Indoor Environment, 2: Offermann FJ. et al.1985, Control of Respirable Particles in Indoor Air with Portable Air Cleaners, Atmospheric Environment, 19: Shaughnessy, Richard J. et al. 1994, Effectiveness of Portable Indoor Air Cleaners: Sensory Testing Results Indoor Air, 4: Sherman MH. and Matson N. 1997, Residential Ventilation and Energy Characteristics, ASHRAE Transactions, 103 (1): Swedish Asthma and Allergy Association (SAAA) 1999, Criteria for Air Cleaners, LRV-1. Thatcher TL. et al. 2002, Effects of room furnishings and air speed on particle deposition rates indoors, Atmospheric Environment, 36: Whitby KT. et al. 1983, Dynamic Model for Evaluating Room-Sized Air Cleaners, ASHRAE Transactions, 89-2A, pp Xu M. et al "Deposition of tobacco smoke particles in a low ventilation room", Aerosol Science and Technology 20: