Permissible Daily Airborne Particle Mass Levels Encompass Brief Excursions to the “London Fog” Range, Which May Contribute to Daily Mortality and Morbidity in Communities
Robert A. Michaels
RAM TRAC Corporation, Schenectady, New York 12309
PM,, levels within the Environmental Protection Agency’s (EPA’s) 24-hour, 150-yg/M® standard have been associated with mortality and morbidity, which remain causally unexplained. EPA’s standard, refocused on PM, ;, has been adjusted downward, assuming that effects observed daily are caused by daily exposures. Real-time particulate matter (PM) monitoring was used to inquire whether 24-hour average PM levels might encompass brief, health-significant excursions. Data from Birmingham, Alabama; Penticton, British Columbia; and Zeebrugge, Belgium, revealed many excursions, one reaching 2000 g/MS or twice the level in the lethal 1952 London fog. Harm from brief exposure to PM in the range of observed excursions was demonstrated experimentally in people, dogs, and rats. In rats, the area of lung surface developing lesions was elevated when the 4-hour average aerosol dose rate included a short-term burst. Elevations were observed with each of two aerosols, and were more pronounced with four excursions than with one excursion. In another study, short-term PM exposure killed rats. In dogs, short-term PM exposure caused serious electrocardiogram changes. In humans, asthma symptoms were associated with PM levels, but the strength of the association increased as PM averaging time decreased from 24 hours to 8 hours to 1 hour. Short-term human response to PM also was evident when 300 people were overcome after wind lofted particles into the air during a 4-day outdoor music festival in Queen Creek, Arizona, in April 1997. These findings support four conclusions: (1) EPA’s decision to change the PM standard may be justified by residual health risks associated with the old standard. (2) The time required for PM to cause adverse effects may be briefer than the time interval over which the effects are observed. (3) Greater risk reduction would appear to be achievable via control of 1-hour PM averages than 24-hour averages. (4) Additional study of PM excursions is needed. MicHaeis, R.A.: PerMissiBle DAILY A\rBoRNE Particle Mass Levels ENCOMPASS BrieF EXCURSIONS TO THE “LONDON Foc” RANGE, WHICH May CONTRIBUTE TO Dalty MORTALITY AND MORBIDITY IN COMMUNITIES. Appt. Occur. ENVIRON. HYG. 13(6):385-394; 1998. © 1998 ATH.
he article reports and updates information presented at the Second Colloquium on Particulate Air Pollution & Hu-
man Health, held in Park City, Utah, in May 1996. At court direction, the U.S. Environmental Protection Agency (EPA)
has reviewed the National Ambient Air Quality Standard (NAAQS) for airborne particles.“-* The old and new NAAQSs limit particulate matter (PM) within a diameter of 10 pm (PM,,) to a time-weighted average mass limit of 150 pg/M? of air over 24 hours, but the new NAAQS also controls 1-hour PM, ; to 65 jtg/M°, which is generally lower than the PM, ; fraction of PMyo at 150 ug/M>.
The increase in stringency was based primarily on epidemiological associations of particle elevations during air pollution episodes with elevated mortality and morbidity in communities (for example, see References 5 through 7). Particle levels during the 1952 London fog, during which some 4000 people died over a period of 10 days, have been estimated at 1200 pg/M3®) (PM,, = 1200 ug/M%). Recent studies associate significant elevations of morbidity and mortality with particle levels well within PM,y = 150 g/M3? (reviewed in Reference 9), and below levels which have been explainable by known toxicological mechanisms. The present study inquires whether 24-hour average particle levels educed via the EPA reference gravimetric monitoring method might encompass undetected shorter-term excursions sufficient to explain the observed adverse effects.
Methods
The method of investigation consisted of evaluating available analytical data and literature to resolve three technical issues. First, the study determined whether instrumentation is available to reliably and economically measure particle levels over short intervals. Second, it quantified short-term excursions of particle mass contributing to 24-hour averages well within the NAAQS. Third, it assessed the possible toxicological adequacy of such excursions, considering their relatively brief persistence, to cause mortality and/or morbidity.
The findings regarding the three issues elucidated above were evaluated relative to EPA policies and procedures for air pollutant standard setting under the Clean Air Act“1) based on the weight of evidence for the existence, nature, and magnitude of public health risks. Ultimately, the findings were evaluated relative to a major regulatory issue facing state health departments and the EPA. That issue is whether adequate public health protection can be secured via the 24-hour NAAQS, which limits particle excursions only to the insignif-
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500 400 : f g 300 iN 2 1-Hour Average PM-10 z 5 200 o z ¢ N Zz 10 5 24-Hour Average PM-10 z “ fo} -100 4 4 — : —— + at +— 30-Jul-94 31-Jul-94 1-Aug-94 2-Aug-94 3-Aug-94 4-Aug-94 5S-Aug-94 6-Aug-94 7-Aug-94 Time
FIGURE 1. Hourly and daily PM,, concentration in Penticton, British Columbia, July 30 to August 7, 1994. Source: Rupprecht et al.
icant degree to which they produce exceedances of 24-hour PM limits.
Findings
Identification of Monitoring Technologies
Two PM monitoring technologies were found to be capable of reliably measuring PM mass levels in real time or over short intervals.@:!2-2 Inertial monitors can measure particles virtually continuously, whereas beta attenuation monitors can do so over intervals of an hour or less under realistic conditions. Both technologies represent alternatives to the reference gravimetric method for monitoring airborne particles.
Airborne Particle Excursions
Data sets were screened to identify those which (1) were obtained using one or both of the identified technologies, (2) exhibited short time resolution, (3) encompassed at least one 24-hour period, and (4) revealed 24-hour airborne particle levels within the current NAAQS. Screening identified three locations and 10 days during which 24-hour average PM, was less than 150 mg/M3. At all three locations, and in six of the ten identified cases meeting the screening criteria, excursions exceeding 150 pug/M® were noted?" (Figure 1, Table 1).
In Birmingham, Alabama, on April 16, 1990, 24-hour average PMj, was 70 we/M?, but two 1-hour average excursions exceeding 200 yg/M? were recorded. In Penticton, British Columbia, on approximately 8 days between July 30 and August 7, 1994, 24-hour average PM; varied from close to 0 to close to 150 g/M?, but excursions exceeding 150 g/M?
were recorded on 4 of the 8 days (Figure 1). These included three 1-hour average excursions to 300 g/M® and two excursions to approximately 500 yg/M°. In Zeebrugge, Belgium, on March 15, 1993, 24-hour average PM, was 101 pg/M3>, but several 15-minute average excursions exceeding 200 yg/M3 were recorded. In one excursion, the 15-minute average reached 2000 g/M%, significantly exceeding the estimated magnitude, though not the persistence, of the 1952 London fog ($1200 prg/M?).
Table 1 summarizes the observed excursions and quantifies their magnitude relative to 24-hour average and London fog particle mass benchmarks. Excursion attained levels from 0.1 to 1.6 times the London fog benchmark. Four of 27 excursions (15%) reached at least 0.5 of the London fog level. These estimates are based on the conservative assumption that the average London fog particle mass estimate consisted of only PM, , whereas PM,, may have represented only a fraction of this estimate.
Toxicological Significance
Numerous studies of short-term effects exerted by PM during controlled acute human exposures were identified. 222 Many of these reported positive results.7?4-47 Table 2 exemplifies the positive studies, ranking the tabulated examples, along with findings of daily mortality studies (for example, see Reference 5) and the current PM;, standard, in increasing order of the product of concentration (c) times exposure duration (t). (Haber’s rule states that c X t = constant; Ten Berge’s adjustment states that c? X t = constant, but the value of n is species specific, and unknown for PM.)
a
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Particulate Matter Excursions 387
TABLE 1. Short-Term Particle Excursions Without Exceedance of the 24-Hour PMj Standard
he RN i A EES
24-Hour Airborne Excursion- ExcursionAverage Particle Excursion to-24-Hour to-1952 Particle Mass Averaging Mass London Fog Reference Mass Excursion Time Ratio Ratio* Location No. Date (ng/M?) (ug/M3) (min) Zeebrugge, Belgium 56 3/15/93 101 1898 15 18.8 1.6 Penticton, British Columbia 19 7/31/94 50 500 60 10.0 0.4 Zeebrugge, Belgium 56 3/15/93 101 780 15 Tal. 0.7 Zeebrugge, Belgium 56 3/15/93 101 648 15 6.4 0.5 Zeebrugge, Belgium 56 3/15/93 101 553 15 535 0.5 Penticton, British Columbia 19 8/1/94 100 460 60 4.6 0.4 Zeebrugge, Belgium 56 3/15/93 101 425 15 4.2 0.4 Zeebrugge, Belgium 56 3/15/93 101 355 15 3:5 0.3 Penticton, British Columbia 19 8/1/94 100 320 60 3.2 0.3 Birmingham, Alabama 13 4/16/90 70 222 60 3.2 0.2 Birmingham, Alabama 13 4/16/90 70 208 60 3.0 0.2 Penticton, British Columbia 19 8/2/94 120 350 60 29 0.3 Penticton, British Columbia 19) 8/1/94 100 280 60 2.8 0.2 Zeebrugge, Belgium 56 3/15/93 101 283 15 2.8 0.2 Birmingham, Alabama 13 4/16/90 70 176 60 2.5 0.1 Penticton, British Columbia 19 8/3/94 120 290 60 2.4 0.2 Zeebrugge, Belgium 56 3/15/93 101 238 15 2.4 0.2 Zeebrugge, Belgium 56 3/15/93 101 230 15 23 0.2 Zeebrugge, Belgium 56 3/15/93 101 220 15 2.2 0.2 Zeebrugge, Belgium 56 3/15/93 101 220 15 22 0.2 Penticton, British Columbia 19 8/2/94 120 250 60 2.1 0.2 Penticton, British Columbia 19 8/3/94 120 240 60 2.0 0.2 Zeebrugge, Belgium 56 3/15/93 101 195 15 1:9 0.2 Zecbrugge, Belgium 56 3/15/93 101 193 15 Ab] 0.2 Zeebrugge, Belgium 56 3/15/93 101 185 15 1.8 0.2 Penticton, British Columbia 19 8/3/94 120 210 60 1.8 0.2 Zeebrugge, Belgium 56 3/15/93 101 153 15 15 0.1 Summary Excursions detected 27: ’ Birmingham, 3/1 day; Penticton, 9/9 days; Zeebrugge, 15/1 day Mean excursion (44g/M?) 99 373: = oe 0.3 Standard deviation (4g/M%) — 17 342 3.6 0.3 Coefficient of variation — 17 91 = 92 91 Locations examined 3 — — = — a Fraction exhibiting 1.00 <a = = excursions Days examined 10 = — — Fraction exhibiting 0.60 — _ = = = excursions
a eed AConservatively assumes London fog = 1200 yg/M®, but this estimate is probably total particulates rather than only PM,o. The deaths of approximately 4000
people over 10 days in 1952 have been attributed to the London fog.
Table 2 indicates that adverse particle-associated health effects have been demonstrated beginning at levels as low as 202 pg/MP47) and at exposure durations as short as 30 minutes.(4647) These effects were documented in asthmatics at rest47) or engaged in exercise.@® Participant ages in these studies varied from 19 to 504% and 23 to 48.47) Thus, no frail elderly, elderly, or infirm subjects were included in the studies. Other positive studies likewise involved asthmatic subjects exercising or at rest, of ages up to 57 years.2935.35.44 For ethical reasons, adverse effects, when observed, could not be allowed to progress to clinical significance. However, reported
effects included increased airway resistance; reduced values of lung function parameters such as forced expiratory volume at 1 and 3 minutes (FEV,, FEV3), forced vital capacity (FVC), and peak flow; as well as such symptoms as throat irritation and coughing.
In addition to the clinical studies described above, in which exposure of asthmatics to PM was controlled, there are two studies in which exposure to PM was uncontrolled. A study conducted by Lawther et al.“®) employed a diary method, in which bronchitis patients recorded daily changes in the severity of their symptoms relative to their recollection of previous-
TABLE 2. Particle Effects Revealed in Controlled Acute Human Exposures Se TEESE INI E IS TINIE R REDE DRI EEE EE Ee He
Reference Healthy Asthmatic Activity Subject Particle Dose x No. Subjects Subjects Level Age Span Type(s) MMAD Dose Duration Duration Effect(s) ) (HM) = (g/M3) (min) (wg min/M®) 47 30 25 At rest 23-48 TSP: 202 30 6060 Healthy: no effects; asthmatic: reduced lung function (FEV1) 46 0 15 Exercise 19-50 NaCl, H,SO,, 0.80 700 30 21,000 Reduced lung function NH, (FEV1) 44 0 17 Exercise 20-57 NaCl, H,SO, 190 120 22,800 Reduced lung function (FEV1) ois} 24 0 Exercise 18-35 Activated carbon + 1.4 1020 120 122,400 Cough, reduced lung formaldehyde function (FVC, FEV3, peak flow) 10 50-100 1440 72,000 Daily morbidity and 144,000 mortality begin 10 150 1440 216,000 NAAQS for PM-10 33 12 0 Exercise 20-39 NaCl + H,SO, 0.9 2000 120 240,000 Throat irritation; no effect on lung function 29 0 12 At rest 25-41 NaCl, 30 mOsm 5.3-6.1 5,900,000 25,900,000 Increased specific airway to resistance 87,100,000
SD
MMAD = mass median aerodynamic diameter.
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Particulate Matter Excursions 389
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strength of association (regression coeff.)
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FIGURE 2. Dependence of PM, /asthma symptom association on PM, averaging time. Adapted from Delfino et al. Figure assumes that
Haber’s rule applies: concentration X time = constant, where n = 1.
day symptom severity. The diary method required participants to make an entry pertaining to a full day of symptom experience. Lawther ef al. reported a clear association of symptom severity with daily concentrations of smoke or sulfuric acid in London. They concluded, however, that this association could not be attributed to the measured 24-hour average concentrations, but more likely to “the effects of brief exposures to the maximum concentrations occurring during the day,” which may be several times the 24-hour average,“ which served merely as a surrogate parameter for such excursions. The basis for the Lawther et al. conclusion included their (uncited) experimental work on normal subjects:
... the measurements quoted in the present paper relate only to 24-hour average concentrations of sulphur dioxide and other pollutants. The evidence that we have from experimental work on normal subjects suggests that the effects of inhaling prepared mixtures of pollutants are of rapid onset, and peak concentrations encountered during the day may therefore be more relevant than 24-hour averages. The effects that we have reported cannot be considered as the result of 24-hour exposures to at least 500 pug/M? of sulphur dioxide together with 250 ug/M? of smoke: they are more likely to reflect the effects of brief exposures to the maximum concentrations occurring during the day, and these may be several times the 24-hour averages.
A second clinical study in which exposure of asthmatics to PM was uncontrolled was conducted by Delfino et al.©° Asthma symptom severity was found to be associated with PM levels. Significantly, the strength of the association increased as PM averaging time decreased from 24 hours to 8 hours to 1 hour. Figure 2 shows that the strength of the association actually increased with increasing exposure duration, as would be expected. However, as also shown, this trend was reversed unambiguously when equivalent exposure metrics were used, specifically, inhaler use per unit of exposure time (per hour) rather than per total exposure time.
A short time frame for human response to PM was also evident when 300 people were overcome shortly after wind lofted fine particles into the air during a 4-day outdoor music festival attended by an estimated 40,000 people in Queen Creek, Arizona, in April 1997. Arizona has a high concentration of residents with respiratory conditions. Examination of data reported by emergency medical personnel operating a treatment station at the concert revealed the approximate time course of visitations to the station (Figure 3). Figure 3 illustrates a significant elevation, indeed a peak, in the rate of visitation for emergency medical treatment, and a similar increase in the rate at which individuals seeking medical attention were transported to area hospitals. Indeed, the rate increased so sharply that ambulances quickly saturated the local hospital, necessitating airlifting of patients to more distant healthcare facilities.
The studies described above demonstrate adverse health effects induced by short-term exposure to particles. However, exposure was routinely terminated to preclude symptoms from progressing to clinical significance. This ethical constraint has been overcome, however, in investigations using animal models. Investigations into airborne particle-induced effects on rats (Figure 4) and dogs (Figure 5), underway at Harvard University’s School of Public Health, have been published in abstract form,®'~59 and more completely in oral and written reports by John Godleski®* to the Clean Air Science Advisory Committee. The findings implicate short-term, possibly electrophysiological mechanisms of fine particle-induced cardiopulmonary malfunctioning, shown via electrocardiograms (EKGs; Figure 5).
Nearing et al.) induced ST segment elevation and periods of apnea in dogs inhaling fly ash at 1000 ug/M® for 3 days, 4 hours/day (Figure 5). Nearing et al. also demonstrated increased T-wave alternans amplitude, indexing vulnerability to ventricular fibrillation, in dogs inhaling fly ash at 3000 g/M?>
390 R.A. Michaels
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—— patients —— hospitalizations
Friday, n= 19/80
BR
a
number of individuals per hour
Saturday, n= 57/300
400 520 640
2,880 000 120
240 360
3,480
3,600
3,720
3,840
3 4 4,200 4,320 4,440 4,560 4 4,800
FIGURE 3. Approximate emergency visit and hospitalization rates, Country Thunder concert, Queen Creek, Arizona, April 24-27, 1997. Assumes reporting period beginning at 12 noon Thursday, April 24, and ending at 8:00 p.m. Sunday, April 27, 1997.
for 3 days, 3 hours/day. ST segment elevation and T-wave alternans effects began soon after exposure and became most prominent by day 3. Apnea for 20 to 30 seconds occurred during hour 4 on day 3 (Figure 5). As a frequent precursor to heart attacks, T-wave alternans represents a clinically serious cardiological event suggestive of PM lethality.
PM lethality also has been demonstrated empirically in animal models. Godleski®* and Godleski et al.©” used rats exhibiting either monocrotyline-induced pulmonary inflam-
— 044 03-2 p> =I Son % S 5 7 281028 yg/euM o4 Reem 0.0- control airborne particle normal monocrotal- regimen disease bronchitis model
FIGURE 4. Mortality induced by 6-hour exposures of rats to airborne particles on three consecutive days. Adapted from Godleski et al.)
mation or sulfur dioxide-induced chronic bronchitis. Rats with respiratory pathology then inhaled particles concentrated from Boston air at 228 to 288 pg/M? for 3 hours/day, 6 hours/day. Breathing rates declined from 17.1/min in controls to 9/min in exposed rats. Apnea for up to 22 seconds occurred. Bronchoconstriction occurred in 80 percent of rats exhibiting respiratory disease, compared with 25 percent of controls. Rats exhibited no signs of distress, but deaths occurred on all exposure days, possibly as a result of particle-induced arrhythmias or other EKG-related cardiopulmonary malfunction. Godleski®* indicated that mortality might also be consistent with systemic effects following particle induction of proinflammatory mediators, such as the cytokines interleukin-1 and tumor necrosis factor-alpha.
Two bioassays were conducted in which aerosols were delivered either constantly at the average concentration, or included excursions. The bioassays, though unpublished, represent results obtained in an investigation by Kleinman et al.,©5) which was published, but excluded from that report, which did not address the issue of excursions. In both bioassays, rats were exposed for 4 hours to either of two aerosols using the published methods.) However, in the unpublished bioassays, the rates at which the aerosols were delivered included either one or four excursions (Figure 6). Excursions were defined as elevations of at least 50 percent above an otherwise constant steady-state aerosol concentration, lasting for at least 5 minutes. Unpublished Kleinman et al. bioassays involved microscopic examination of rat lungs 48 hours after 4-hour controlled exposures of groups of six rats to either of
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Particulate Matter Excursions 391
07. | 10: 2X 06. re ae os 3 Fy 9. a8 : Sel; os Ff 03. ce ad eS gel 6 § = 02. : 5 $ 5 i : igs BRS ‘ oe Es] 4 control 1,680 pg/cu M 3,040 yg/cu M bw | i — airborne particle regimen 2B 4 = € LE Fi & 5 AMS, zero S25. : excursions, s a ie n= 6 rats B 2 3 n=6rats SAM, zero = 15 —4 excursions, 5 n= 6rats SAM, one B excursion, 8 os. : n=6rats é - OF = mi © 2e) be congnal 1,680 pg/cu M 3,010 ug/eu M FIGURE 6 Pulmonary effect of 5- to 10-minute sulfate aerosol ex: . " 7 cursions during 4-hour controlled exposures of rats. AMS, ammoairborne particle regimen
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airborne particles on three consecutive days. Adapted from Nearing et al, 8253)
two sulfate aerosols, designated AMS and SAM, with the compositions shown in Table 3.
The fraction of microscopically examined rat lung area exhibiting aerosol-induced lesions was evaluated in groups of rats exposed to excursions versus no-excursion controls exposed to the same aerosol. Thus, except for the excursions, treated and control rats experienced identical exposures. Results are presented in Figure 6, which consists of two sets of paired columns. In both sets, the column to the right indicates greater lesion area in rats exposed to aerosol excursions. However, in the case of AMS, the increment over control rats was greater, reflecting the occurrence of four excursions, compared with only one excursion in the case of rats exposed to the SAM aerosol.
Regulatory Significance
The findings reported in bioassays, controlled human exposure studies, and epidemiology studies in which PM exposure is uncontrolled must be evaluated relative to EPA policies and procedures for air pollutant standard setting!" based on the weight of evidence for the existence, nature, and magnitude of public health risks. The EPA requested the American Thoracic Society (ATS) to define toxicological effects and degrees of
nium, iron + manganese sulfates (0.5, 1.0, 0.2 mg/M); SAM, sulfuric acid, iron + manganese sulfates (0.5, 1.0, 0.2 mg/M®); all, ozone, nitrogen dioxide, sulfur dioxide (0.6, 2.6, 5.0 ppm). Data source: Kleinman, M., unpublished.
effect which might qualify as forming the basis for limiting air pollutants, and both ATS® and EPA“ have published these guidelines.
According to EPA policy,” air pollution limits must be set based on a clinically significant adverse effect, not any effect, as defined by ATS:“° “We define ‘adverse respiratory health effects’ as medically significant physiologic or pathologic changes.”
Many of the controlled human exposure studies focus on asthmatics during exercise and at rest. These studies, described earlier in Table 2 and associated text, reveal symptoms of asthma exacerbation such as bronchoconstriction, increased airway resistance, reduced FEV, and FEV;, reduced FVC, and reduced peak flow (for example, see Figure 7). The issue of whether these effects can form the basis for air pollutant standard setting under EPA’s Clean Air Act implementation policies and procedures is resolved by ATS:"
the eye, nose, and throat irritation associated with... urban ‘smog’ or photochemical oxidant air pollution is not medically important and is, therefore, not considered an ‘adverse health efect’ in relation to the Clean
TABLE 3. Composition of Sulfate Aerosols AMS and SAM
Component AMS SAM Varied components Ammonium 0.5mg/M? 0.0mg/M> Sulfuric acid 0.0mg/M? 0.5mg/M> Invariant components Iron sulfate 1.0mg/M? 1.0mg/M? Manganese sulfate 0.2mg/M? 0.2mg/M* Nitrogen dioxide 2.6 ppm 2.6 ppm Ozone 0.6 ppm 0.6 ppm Sulfur dioxide 5.0 ppm 5.0 ppm
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respiratory conductivity, FEV,,/FVC, FEV, provacative cum. dose, men women men women men women men women [10/(cmH,0/L.s)| 5) Giters) (methacholine units) clean air control 11 12 0.825 0.797 3.01 2.90 55 47 Taipei tunnel air** 07 08 0.773 0.741 2.78 2.69 3.6 33 statistical sis 0 7 istical significance <0.001 <0.001 <0.05 <0.05 <0.05 <0.05 <0.001 <0.001 Provacative cum. dose, women (methacholine units) 40-4 provacative cum. dose, men (methacholine units) g FEV1.0, women (liters) i=l FEV1.0, men (liters) aa FEV1.O/FVC, women (..) 20-4 FEV1.0/FVG, men (..) 104 respiratory conductivity, women (10/(cmH20/L.s)] 00-4 respiratory conductivity, men (10/(amH20/L.s)] ean Taipel control nnel a Adapted from: Yang, S.C.; and SP. Yang. Respiratory function changes from inhalation of polluted air. Archives of Environmental Health, 49(3):182-7, May/June 1994. **treatments: suspended particulates (ug/cu M)|__CO (ppm) SO, (ppb) NO, (ppb) clean-air control | 34 4/-9 1.6+/-07 84/3 46 +/-15 Taipei tunnel air 202 +/-33 3.44/18 112 +/-19 488 +/-24
FIGURE 7. Effects of breathing polluted Taipei tunnel air on asthmatics at rest. Adapted from Yang and Yang.“¢”
Air Act. This irritation certainly is annoying or bothersome, which puts it in the category of an effect on human welfare. However, irritation that triggers an asthmatic attack is an adverse health effect. [emphasis added]
Clearly, the symptoms evoked by controlled clinical exposures as described and tabulated above (Table 2) do not include triggering an asthmatic attack. However, according to EPA and ATS guidelines, this clinical endpoint need not be (and for ethical reasons, cannot be) experimentally induced to be used in standard setting. Rather, the guidelines justify inferring greater severity of response with greater exposure. ATS continues: “This spectrum of biologic responses would be evoked more strongly as the concentrations of pollutants are increased.”
Conclusions Excursions of airborne particle mass may elevate PM, by an order of magnitude relative to 24-hour average levels within
the 150-~g/M> NAAQS. PM levels comparable to those reached during documented excursions can exert toxicological effects upon humans and animals, including mortality in animals. Such excursions might constitute a previously unrecognized cause of mortality and morbidity revealed by epidemiology studies based on 24-hour average PM__ levels. Toxicological effects reported in association with PM in the range of observed excursions are of types determined to be appropriate for air pollutant standard setting under the Clean Air Act, according to EPA criteria. Thus, EPA’s decision to change the PM standard may be justified by residual health tisks associated with the old standard. However, the time required for PM to cause adverse effects may be briefer than the time interval over which the effects commonly have been observed. A corollary is that threshold effects exerted by PM may be caused by brief exposure to PM excursions exceeding effect thresholds. Such thresholds may be imperceptible, how-
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ever—not because several noncancer effects lack thresholds if they are caused by PM, but because the thresholds may be obscured by dilution within 24-hour PM averages routinely reported.
Thus, EPA’s preference for managing 24-hour average PM concentrations, reflecting the availability of 24-hour average PM data, is technically unjustified. Additional study of the epidemiology of PM excursions is needed. However, data already available and presented herein suggest that greater reduction of residual risks posed by PM within the old NAAQS may be achievable via control of 1-hour PM averages than by control of 24-hour averages.
In many if not most industries, controlling PM excursions to a 1-hour mass limit of, say, 300 g/M? can be achieved more economically than controlling 24-hour averages, because industrial processes often include brief periods of reduced PM control, when excursions occur. For example, plastics factories may produce several types of plastics using the same (extrusion) equipment operated with resins at different temperatures. Shifting to a new plastic type requires purging the previous resin, changing the temperature, and running the equipment with the new resin. PM emissions during these transitional phases of operation can be controlled more economically because they require improvements to only a small part of overall operations. For example, plastics factories can run resins requiring higher temperatures after first running lower temperature resins, or they can assure adequate extruder temperature reduction prior to introducing lower temperature resins.
Acknowledgment This research was supported by Rupprecht & Patashnick, Inc., Albany, New York.
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Publication citation
Michaels, RA. Permissible daily airborne particle mass levels encompass brief excursions to the ‘London fog’ range, which may contribute to daily mortality and morbidity in communities. Applied Occupational and Environmental Hygiene, 13(6):385-94, June 1998;