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PTR-MS Assessment of Photocatalytic and Sorption-Based Purification of Recirculated Cabin Air during Simulated 7-h Flights with High Passenger Density A R M I N W I S T H A L E R , * ,† PETER STRØM-TEJSEN,‡ LEI FANG,‡ TIMOTHY J. ARNAUD,§ ARMIN HANSEL,† T I L M A N N D . M A¨ R K , † A N D DAVID P. WYON‡ Institut fu ¨ r Ionenphysik, Leopold-Franzens-Universita¨t Innsbruck, A-6020 Innsbruck, Austria, International Centre for Indoor Environment and Energy, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark, and Boeing Commercial Airplane Group, P.O. Box 3707, MC 02-WH, Seattle, Washington 98124
Four different air purification conditions were established in a simulated 3-row 21-seat section of an aircraft cabin: no air purifier; a photocatalytic oxidation unit with an adsorptive prefilter; a second photocatalytic unit with an adsorptive prefilter; and a two-stage sorptionbased air filter (gas-phase absorption and adsorption). The air purifiers placed in the cabin air recirculation system were commercial prototypes developed for use in aircraft cabin systems. The four conditions were established in balanced order on 4 successive days of each of 4 successive weeks during simulated 7-h flights with 17 occupants. Protontransfer reaction mass spectrometry was used to assess organic gas-phase pollutants and the performance of each air purifier. The concentration of most organic pollutants present in aircraft cabin air was efficiently reduced by all three units. The photocatalytic units were found to incompletely oxidize ethanol released by the wet wipes commonly supplied with airline meals to produce unacceptably high levels of acetaldehyde and formaldehyde.
On modern aircraft, outside air typically makes up only half of the total air supply per passenger while the remaining air is filtered recirculated air (2). This ventilation system design provides improved air distribution in the cabin and allows the engine to use less fuel for air supply and pressurization. Most filtration systems on board modern aircraft use only high-efficiency particulate air (HEPA) filters to remove airborne particulates while odors and potentially irritating volatile organic compounds (VOCs) are not removed from the airstream. A variety of air purification techniques including sorptionbased purification on activated-carbon-based filters, UVphotocatalytic oxidation, plasma oxidation, and thermal oxidation have been developed for VOC filtration. Photocatalytic oxidation is at first sight a strong candidate due to low airflow resistance, low-energy requirement, and low total weight. Titanium dioxide (TiO2) is known to be one of the most effective photocatalysts for the degradation of organic pollutants. To initiate redox reactions at the catalyst’s surface which are capable of destroying organic contaminants, the catalyst surface is irradiated with UV light. Different approaches such as the preparation of supported photocatalysts, doping of TiO2 with other transition metals, or coupling of two different semiconductors can be used for improving the photocatalytic activity. A large number of photocatalytic oxidation studies have been conducted (3, and references therein). However, few are directly relevant with respect to practical use of these devices in occupied buildings and aircraft. The dominant organic air contaminants in an occupied aircraft cabin environment under normal operating conditions are a mixture of human bioeffluents as well as emissions from the cabin interior components, from food and beverages, and from cosmetic or personal care products (4). The present study examined the ability of two prototype air cleaners from different manufacturers, both comprising an adsorbent unit in combination with a photocatalytic oxidation unit, to purify the recirculated air in an aircraft cabin environment, in simulated 7-h flights with passengers present at an occupant density corresponding to Economy Class. A state-of-the-art sorption-based filter was included in the study to provide a direct comparison with a well-established air filtration technology. On-line proton-transfer reaction mass spectrometry (PTR-MS) based analysis of volatile organics in air constitutes the dependent variable in this report.
Introduction
Methods
The environment in an aircraft cabin differs in several ways from the indoor environments in offices and homes, due mainly to the much higher occupant density (up to 1.7 passengers/m2 floor area), the low relative humidity (usually 10-20% RH), and the low air pressure (as low as threequarters of sea level air pressure at typical cruising altitude). From 1996, the U.S. Federal Aviation Administration (FAA) required 0.25 kg/min per person of outside airflow to an aircraft cabin, corresponding to a volumetric flow rate of 3.5 L/s per person at sea level, which is higher than the requirements specified for buildings with high occupant density (such as auditoriums and conference rooms) by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) (1).
Description of the Simulated Aircraft Cabin. The air sampling reported in this paper took place in the environmental control system of a simulated aircraft cabin containing 21 seats (3 rows of 7). The cabin (width, 4.9 m; length, 3.2 m; cross-sectional area, 8.9 m2; volume, 28.5 m3) was installed inside an existing climate chamber. The cabin interior components (seats, carpet, and wall panels) were all used aircraft parts. Shape, volume, appearance, radiant temperature distribution, and air circulation patterns of the simulated cabin closely resembled that of a 3-row section of a Boeing 767 aircraft. A more detailed description of the simulated aircraft cabin can be found in Wisthaler et al. (5). The total supply airflow to the cabin, including recirculated air, was 200 L/s, of which 40.4 L/s was outdoor air. With 17 subjects present in the cabin this corresponds to an outdoor air flow rate of 2.4 L/s per person at sea level, which is lower than the minimum supply of 3.5 L/s per person at sea level required by U.S. FAA regulation. The outdoor air supply rate used here is in rough conformance with ASHRAE Standard
* Corresponding author phone: +43 512 507 6249; fax: +43 512 507 2932; e-mail:
[email protected]. † Leopold-Franzens-Universita ¨ t Innsbruck. ‡ Technical University of Denmark. § Boeing Commercial Airplane Group. 10.1021/es060424e CCC: $37.00 Published on Web 11/23/2006
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62 (1) which contains a minimum outdoor air intake requirement of 2.5 L/s per person for facilities with high occupant density. Recommendations from ASHRAE and the demand to reduce fuel costs may lead to the introduction of changes to standards in relation to aircraft air quality and it was our intent to study the effects of increased pollutant levels as well as the performance of the air purifiers under these conditions. We do not consider the lower outdoor air supply rate used here to be critical for the main findings presented herein. Outside air was cooled, purified, and dehumidified so that its humidity content corresponded to that of outside air at altitude, i.e., at -50 °C. A HEPA filter that had been in service in a passenger aircraft for 18 months was installed in the air recirculation system upstream of any air cleaners. Ethylene glycol is used in de-icing procedures and as a result is sometimes present in airplane cabin air. Trace amounts of the ethylene glycol that had once leaked from a cooling coil on the roof of the climate chamber were still present on the cabin carpet during the experiments reported here. Experiments were conducted at ground-level barometric pressure. Cabin air temperature averaged 23.2 °C ((0.1 °C), the average value of cabin air relative humidity was 21% ((2%). In the absence of occupants the relative humidity fell to about 5%. It should be noted that several classes of contaminants potentially present aboard commercial airliners (e.g., gasphase products of ozone-initiated chemistry, lubricants, pesticides, and hydraulic fluids) were not present in the cabin environment simulated here. Experimental Conditions. On four successive days, four different conditions were established in the simulated aircraft cabin, i.e., with the following air purification units being placed in the recirculation air system: (1) no purifier (no); (2) a prototype air cleaner from manufacturer 1 comprising an adsorbent unit and a photocatalytic unit located downstream from the adsorbent unit (PCO1); (3) a prototype air cleaner from manufacturer 2 also comprising an adsorbent unit and a photocatalytic unit located downstream from the adsorbent unit (PCO2); (4) a state-of-the-art sorption-based filter (gas-phase absorption and adsorption, GPA). These conditions were established in balanced order on 4 successive days of 4 successive weeks (denoted as w1, w2, w3, and w4). Seventeen passengers entered the simulated cabin section at 09:00 on each experimental day and remained in the cabin until 16:00. A standard cold airline lunch was served each day 11:45-12:15. Biscuits and nonalcoholic beverages were freely available throughout each flight. Air Cleaners. The photocatalytic air cleaners were prototypes from suppliers to the airplane manufacturing industry, who for commercial reasons will not permit their names to figure in this report and will not release full specifications of their products. The prototypes were specially dimensioned for the present study; results from a similar (unpublished) study carried out previously had been implemented in their design. Both air purifiers comprised an adsorbent unit located upstream from the photocatalytic unit. An adsorbent prefilter spares the photocatalytic unit from receiving an unmanageably high pulse of pollutants and subsequently delivers the pollutants to the photocatalytic unit at a manageable rate via equilibrium-driven desorption. The face dimensions of the PCO1 unit were 52 × 61 cm, resulting in a face velocity of 50.3 cm/s. The air cleaner comprised a 15 cm long activated-carbon-based adsorptive stage followed by a 70 cm long PCO unit. The latter consisted of rows of 36 W UV-C lamp pairs sandwiched between a series of panels positioned perpendicular to the gas stream. Each panel was composed of several expanded aluminum 230
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sheets which defined a plurality of apertures through which the gas flowed. Panels were coated with a modified TiO2 catalyst, designed to have improved conversion at low to moderate relative humidity. According to the producer, this geometric design has the advantage of removing the angular dependence of the illuminated surface area typical of monolithic designs. Since both the surface area of the photocatalytic agent exposed to UV light and the area in contact with the air to be oxidized increase with such a design, a gain in oxidation efficiency was expected. The PCO2 unit also comprised a photocatalytic unit and an adsorptive prefilter. The overall unit dimensions were 49.8 × 49.8 × 53.3 cm, resulting in a face velocity of 64.3 cm/s. The 2.54 cm thick adsorptive prefilter consisted of modified activated alumina (Selexsorb CDX, formerly Alcoa World Alumina LLC, Pittsburgh, PA; now Engelhard Corp., Iselin, NJ). Selexsorb CDX is specially formulated for the adsorption of polar organic compounds. The photocatalytic oxidation reactor comprised five banks of UV-C lamps sandwiched between a series of six TiO2-coated, 2.54 cm thick honeycomb monoliths. Three UV lamps per stage generated an average UV-light intensity of 5.5 mW/cm2; each UV light stage was 7.6 cm thick. The GPA unit was a panel filter with axial flow configuration. The panels consisted of a fluted substrate coated with adsorptive media. The filter face dimensions were 50.8 × 50.8 cm, resulting in a face velocity of 61.8 cm/s. Two sorptive stages were implemented, a 10.2 cm deep physisorptive stage followed by a 5.1 cm deep chemisorptive stage. The detailed physicochemical properties of the two sorptive stages were not disclosed by the manufacturer. Each air cleaner was installed in turn in the recirculated air downstream of the used HEPA filter within an hour of the end of the preceding simulated flight. The air cleaners were thus flushed with air from a normally ventilated but unoccupied cabin for about 15 h immediately prior to each simulated flight. Ethanol Challenge. Following the 7-h simulated flight experiments the air-cleaners were challenged in a brief followup study. Ten of the wet wipes (Seton Healthcare Group plc, Oldham, UK) that accompanied the airline lunch were opened and unfolded all at the same time and exposed on the unoccupied seats of the simulated cabin until they were dry. This was done on 3 successive days with the three different air purification units in use. PTR-MS Measurements. The analytical technique used in this study was PTR-MS, which is a chemical ionization technique based on proton-transfer reactions from H3O+ ions to gaseous organic analytes with a higher proton affinity than water (6). The PTR-MS instrument was set up, operated, and calibrated as described in detail by Wisthaler et al. (5).
Results Operation of the Air Cleaning Units. The matrix of VOCs in the simulated cabin environment is inherently complex. Organic air contaminants are a mixture of human effluents as well as emissions from the cabin interior components, from food and beverages, and from cosmetic or personal care products. This complexity is reflected in the mass spectra of cabin air produced by PTR-MS. For the interested reader, examples of PTR-MS spectra for different pollution conditions are given as Supporting Information. In an attempt to give a compound-specific quantitative analysis, we have focused on the eight most abundant detected ion signals that could be assigned to seven major organic pollutants present in the simulated cabin environment. These were, in order of decreasing concentration, ethanol (m/e ) 47), monoterpenes (m/e ) 81, 137), acetaldehyde (m/e ) 45), methanol (m/e ) 33), acetone (m/e ) 59), formaldehyde (m/e ) 31), and isoprene (m/e ) 69).
FIGURE 1. Time evolution of the seven major organic pollutants and the sum of all minor VOCs (TMVOC) detected by PTR-MS in the cabin outflow over a 4-day period in which four different conditions (PCO2, GPA, no, and PCO1) were established (w2 data). Figure 1 shows a typical time evolution of pollutant levels in the cabin over a 4-day period of the experiment during which the four different conditions were established (data are from w2). Before going into detail it is worth highlighting two distinct features in the observed time patterns: human bioeffluents (e.g., acetone or isoprene) quickly reach a steady-state level (∼30 min after the passengers have entered the cabin) and remain approximately constant throughout the simulated flight, whereas other compounds (such as ethanol or monoterpenes) show high-concentration spikes in the nopurifier condition when food is served and/or when wet wipes for hand cleaning are used by the passengers. Strikingly, for both PCO conditions the ethanol increase was accompanied by a spiking increase of acetaldehyde andsto a minor extents of formaldehyde. In the no-purifier condition and with the GPA unit in use no concurrent acetaldehyde and formaldehyde spikes were observed. Basic descriptive statistics as well as graphical displays (in the form of box-and-whisker plots) were used to summarize the data obtained for the four conditions (PCO1, PCO2, GPA, and no). Figure 2 is a box-and-whisker plot showing the median volume mixing ratio as a horizontal bar in the interior of each box, the lower and upper quartile as boxes, and the 5th and 95th percentile as whiskers. The statistical values were calculated for the period 09:45 h (i.e., when steady state was reached) to 15:45 h. Ethanol was the most abundant organic pollutant detected in the cabin. Ethanol is a human bioeffluent (7, and references therein) but the major source in this test was evaporation
FIGURE 2. Box-and-whisker plots showing the median volume mixing ratios as a horizontal bar in the interior of each box, the lower and upper quartiles as boxes, and the 5th and 95th percentiles as whiskers. The figure summarizes the levels of the seven major organic pollutants and the sum of all minor VOCs (TMVOC) measured for the four conditions (PCO1, PCO2, GPA, and no) established on 4 successive days of 4 successive weeks (w1, w2, w3, and w4). Ethanol*: a formic acid interference is discussed in the Supporting Information; acetaldehyde*: see text for ethylene glycol interference. from the wet wipes supplied with the airline lunch. Alcoholic beverages were not served. The simultaneous use of several wipes resulted in peak ethanol levels of several ppmV’s; median ethanol levels were in the 50-250 ppbV range for the no-purifier condition. As discussed in the Supporting Information, formic acid (detected like ethanol at m/e ) 47) may be present in significant amounts for both PCO conditions and the stated ethanol levels should be taken as upper limits. For the GPA and the no-purifier condition, formic acid levels are believed to be in the low ppbV range, causing negligible interference with 1-3 orders of magnitude higher ethanol levels. Median ethanol/formic acid levels were in the 18-22 ppbV range with the PCO2 unit in use; the GPA unit reduced peak ethanol levels (95th percentile: 230-480 ppbV), but subsequent re-emission resulted in increased median levels (130-280 ppbV). The PCO1 unit reduced peak ethanol levels and the median levels were similar to those observed with no purifier in use. Monoterpenes are isomeric C10H16 hydrocarbons that produce two major product ion signals at m/e ) 81 and m/e )137 in the PTR-MS. As can be seen in Figure 2, all three air cleaners reduced both peak and median monoterpene levels very efficiently. Acetaldehyde is an intermediate photocatalytic oxidation product of ethanol and high acetaldehyde levels were observed simultaneously with high ethanol levels when the PCO units were in use. Both PCO units produced very high acetaldehyde levels in the cabin, with the 95th percentile in the 230-670 ppbV range. At levels of tens of ppbV, a signal VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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interference from ethylene glycol becomes dominant. In the PTR-MS ethylene glycol dehydrates upon protonation, forming an m/e ) 45 ion signal, which interferes with the product ion signal from acetaldehyde. Ethylene glycol was accidentally present in the cabin at ppbV levels, as stated earlier. m/e ) 45 median levels in the 28-34 ppbV range were found in the no-purifier condition. Unpublished PTRMS and GC-MS measurements performed under similar conditions in earlier experiments without ethylene glycol contamination yielded acetaldehyde levels in the 4-7 ppbV range, suggesting that during the experiments presented here ∼25 ppbV of ethylene glycol were present in the cabin air in the no-purifier condition. The GPA unit reduced median ethylene glycol/acetaldehyde levels in the cabin down to ∼10 ppbV. Methanol is a human bioeffluent (7, and references therein) but methanol peaks also occurred when food was served. Additionally, it was observed as an incomplete photocatalytic oxidation product of ethanol when the PCO1 unit was in use, probably as a result of surface-associated oxidation chemistry. As a consequence, peak methanol levels were significantly increased for the PCO1 condition. Methanol was efficiently reduced only by the PCO2 unit. The GPA unit was inefficient for methanol removal. Acetone is a major organic constituent in exhaled human breath (7, and references therein). The GPA unit efficiently reduced acetone levels, while the two PCO units were less efficient. Acetone is an abundant indoor contaminant but is also a typical intermediate oxidation product. It is thus likely to be consumed as well as produced by photocatalytic oxidation when applied to complex VOC mixtures. In the PCO1-w3 experiment high acetone levels were observed during/after lunch. The reason for these abnormally high levels remains unexplained. Formaldehyde is an intermediate product in gaseous ethanol photocatalytic oxidation. Both PCO units produced high levels of formaldehyde in the cabin environment, with the 95th percentile in the 20-38 ppbV range. With the GPA unit in use peak and median formaldehyde levels were similar to those obtained in the no-purifier condition. Isoprene is a major organic constituent in exhaled human breath (7, and references therein). Median isoprene levels were in the 9-11 ppbV range for the no-purifier condition. All three air purifiers substantially reduced isoprene levels in the cabin environment, with median levels being in the 2-3 ppbV range. As seen in the spectra given as Supporting Information, a series of minor ion signals were present in the PTR-MS spectra for different pollution conditions. The identity of these signals is unclear; the sum of all these minor VOC signals was designated as “Total Minor VOCs” (TMVOC); time evolution and statistics were included in Figures 1 and 2. All three air cleaners reduced TMVOC significantly. Levels were somewhat higher for the PCO1 unit because it also generated some minor incomplete ethanol oxidation products (see Supporting Information). Ethanol Challenge Experiment. The purpose of the ethanol challenge experiment was to remove the variance due to the completely optional and unrecorded use of wet wipes by subjects during the simulated flight exposures. Figure 3 shows the mixing ratios of ethanol, acetaldehyde, and formaldehyde measured in the cabin outflow when 10 wipes were simultaneously opened and left in the unoccupied cabin until they were dry. This was done on 3 successive days with the three different air purification units in use. Ethanol was the dominant (>99%) primary emission from the wet wipes; monoterpenes (probably limonene) were only observed in traces. The GPA unit rapidly reduced peak ethanol levels and then became a low-level source of ethanol due to re-emission; no aldehydes were observed. Operation of PCO1 232
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FIGURE 3. Time evolution of ethanol, acetaldehyde, and formaldehyde as measured during the ethanol challenge experiment, i.e., when 10 wet wipes were opened at 12:00 and unfolded onto 10 seat trays in the empty cabin. Ethanol*: a formic acid interference is discussed in the Supporting Information. and PCO2 led to the formation of acetaldehyde and formaldehyde, with peak acetaldehyde levels of ∼1.6 ppmV for both units and peak formaldehyde levels of 120 and 70 ppbV, respectively. The challenge experiment confirmed that the formaldehyde and acetaldehyde spike events observed during the simulated flights originated from incomplete oxidation of ethanol evaporated from the wet wipes provided with the airline lunch. Minor intermediate oxidation products observed during the ethanol challenge experiment are discussed in the Supporting Information. Peak and Mean Reductions Achieved by the Air Purifiers. In the first part of the Results section we gave a compoundspecific quantitative analysis of the most abundant VOC species that were detected by PTR-MS in the simulated cabin environment. However, these species (e.g., acetone or isoprene) may be of minor relevance to perceived cabin air quality; minor species may be more relevant in this context. As described above, a large number of minor peaks were found in the PTR-MS spectra that could not be identified. PTR-MS offers the possibility to monitor their relative changes as a function of the air purification unit in use. A comprehensive analysis of observed changes will now be given. Six-hour mean concentrations [X]i were calculated for all detected PTR-MS signals in the period 09:45 h (i.e., when steady-state was reached) to 15:45 h during the simulated flights, for each of the four conditions ([X]i,PCO1, [X]i,PCO2, [X]i,GPA, [X]i,no), together with the maximum of the 15-min running mean ([Y]i,PCO1, [Y]i,PCO2, [Y]i,GPA, [Y]i,no) in that period. Forty-eight signals exceeded a 0.1 ppbV cutoff level for the 6-h mean, i.e., i ) 1, ..., 48. These included the 8 signals due to the major species reported above but excluded their 13C isotopes, hydrates, and minor fragments. For each of these signals the ratio between purifier and no-purifier condition was calculated, both for mean and peak exposure: [X]i,PCO1/ [X]i,no, [X]i,PCO2/[X]i,no, [X]i,GPA/[X]i,no and [Y]i,PCO1/[Y]i,no, [Y]i,PCO2/ [Y]i,no, [Y]i,GPA/[Y]i,no. Relative cumulative frequency distributions of these ratios were derived for each week of experiments. Finally, the 4 week mean and the standard deviation (shown as error bars) of the relative cumulative
A slight aging effect was observed for all units during the course of 7 h simulated flights which may be explained by the progressive desorption of pollutants during the later stage of the flight. A more detailed discussion of the observed aging effect is given as Supporting Information. No systematic decrease in performance was observed during the 4 weeks of operation for any of the air purifiers. Both photocatalytic units were challenged in an additional follow-up study in which unburned jet fuel fumes were released into the unoccupied cabin. The experimental details and results are given in the Supporting Information. Here we just want to point out that both photocatalytic units efficiently filtered jet fuel vapors and that no major formation of intermediate oxidation products was observed.
Discussion
FIGURE 4. Relative cumulative frequency diagrams of concentration ratios between purifier and no-purifier condition for all detected PTR-MS signals (6-h mean and 15-min maxima). Details on how these data were calculated are explained in the text. frequency was calculated for each bin and plotted in Figure 4. The figure may be interpreted as follows: With the PCO1 unit in use the 6-h mean of 78 ( 2% of all detected signals were reduced by a factor of at least 2, and 47 ( 7% of the signals decreased to e30% of the levels observed for the nopurifier condition. For peak exposure the reduction was slightly less efficient. With the PCO2 unit in use 83 ( 5% of all detected 6-h mean signals were reduced by a factor of at least 2, and 41 ( 12% of the signals decreased to e30% of the levels observed for the no-purifier condition. The performance characteristics were thus similar for both PCO units. Removal efficiency tended to increase slightly with increasing molecular weight of the pollutant (data not shown). Isoprene and monoterpenes were identified among the signals with high removal efficiency, suggesting good filtration performance for pure hydrocarbon species. For signals that are typically associated with oxygenated species (e.g., ethanol, acetone, or butanone), the ratio between purifier and nopurifier condition was in the 0.5-0.7 range (i.e., the reduction was by less than a factor of 2). With the PCO1 unit in use increased median levels of acetaldehyde, formaldehyde, methanol, and acetic acid (or hydroxy-acetaldehyde) were observed in all 4 weeks; PCO2 operation resulted in increased median levels of acetaldehyde and formaldehyde. The GPA unit gave the best performance, both in mean and peak level reduction. About 88 ( 5% of all detected signals were reduced by a factor of at least 2, and 57 ( 9% of the signals decreased to e30% of the levels observed for the nopurifier condition. Low removal efficiencies were found for only a few low molecular weight compounds: formaldehyde, methanol, acetonitrile, and an unidentified species at m/e ) 41. Median concentration ratios between purifier and nopurifier condition (of all 6-h-mean signals) during the 4 weeks were 0.32, 0.32, and 0.27 for PCO1, PCO2, and GPA, respectively. These reductions are equivalent to an additional outdoor air supply rate of 5.1, 5.0, and 6.3 L/s per person, respectively.
Photocatalytic air purifiers combined with adsorbent prefilters efficiently removed VOCs in human bioeffluents. However, an apparently trivial event, the simultaneous opening of wet wipes for hand cleaning, resulted in the formation of acetaldehyde levels that are unacceptable on at least two grounds: (1) the odor threshold of acetaldehyde is 0.05 ppmV (8) and (2) there is evidence that exposure to low levels of acetaldehyde constitutes a potential hazard to human health. Short-term exposure to acetaldehyde causes eye, skin, and respiratory tract irritation, while long-term exposure may cause cancer and may also affect the upper airway, red blood cells, kidneys, and growth (9). While official guidelines for short-term exposure to acetaldehyde have not yet been developed, the 1-h maximum acetaldehyde levels in the 200-600 ppbV range observed with the PCO units in use give rise to health concern. Exposure to formaldehyde can cause both short-term irritant effects and long-term health effects such as cancer. Short-term (
