Air quality trends
From 1990 to 2010, annual average PM2.5 in the model decreases
significantly in the eastern US (Fig. 1c), but slightly decreases or even
increases in the northwest, southwest and west (Fig. S2 and Table S3; also
see Fig. S3 for the US 9 regions defined by the National Oceanic and
Atmospheric Administration, Zhang et al., 2016). The dramatic decreasing
trends of PM2.5 in the eastern US were also reported in previous
studies (Gan et al., 2015; Xing et al., 2015) due to emission reductions. The
increasing trend in the western central area is due in part to frequent
wildfires (Dennison et al., 2014; Hand et al., 2013, 2014; Jaffe et al.,
2008; Murphy et al., 2011; Spracklen et al., 2007). In general, the decadal
decreasing trends in the east are larger than
2 µg m-3 decade-1
from 1990 to 2010, especially in the central area
(-3.48 µg m-3 decade-1) and northeast
(-3.14 µg m-3 decade-1). The summertime average of
1 h daily maximum O3 decreases significantly in the central and
eastern US, generally at a rate greater than 4 ppbv decade-1. It also
decreases in the western US, but at a much smaller rate than in the east,
generally less than 1 ppbv decade-1 (Fig. 1f; Table S3).
In Fig. 2, both the spatial average and population-weighted average (PWA)
annual PM2.5 exhibit smooth decreasing trends (Fig. 2, top): the
spatial average of annual PM2.5 has decreased by 29 %, from
9.07 µg m-3 in 1990 to 6.45 µg m-3 in 2010,
with a decadal rate of decrease of 1.1 µg m-3 decade-1.
The corresponding PWA PM2.5 decreases by 39 %, from
17.61 µg m-3 in 1990 to 10.73 µg m-3 in 2010,
with a decadal decreasing rate of 3.2 µg m-3 decade-1.
Years with high PM2.5, such as in 1994, 1996, and 2000, are mainly
caused by increases in organic carbon due to large wildfires in the western
US (Spracklen et al., 2007). Both the spatial average and PWA O3 also
exhibit decreasing trends over the past 2 decades, with greater inter-annual
variability resulting from meteorological variability (Porter et al., 2017).
The spatial average O3 concentration decreases by 9 %, from
55.02 ppbv in 1990 to 49.99 ppbv in 2010, decreasing at a rate of
2.4 ppbv decade-1. The PWA O3 also decreased by 9 %, from
58.96 ppbv in 1990 to 53.57 ppbv in 2010, decreasing at a rate of
3.0 ppbv decade-1. We also calculate the air quality and mortality
burden trends separately for two 11-year periods, 1990 to 2000 and 2000 to
2010, following Astitha et al. (2017). Both PM2.5 and O3
decrease more strongly in the second decade than in the first decade for both
spatial average and PWA (Table S5), consistent with previous findings
(Astitha et al., 2017; Gan et al., 2015; Porter et al., 2017; Xing et al.,
2015).
Population-weighted average (Popweighted avg) and spatial
average over CONUS land areas of annual average PM2.5 (a) and summertime average of 1 h daily maximum
O3 (b) concentration from 1990 to 2010.
Population-weighted average concentrations are based on population in each
year. Using the same population in each year yields estimates of
population-weighted concentrations that are only slightly different (not
shown).
We then calculate trends in the number of days annually that exceed the daily
PM2.5 standard (35 µg m-3), and the daily MDA8
O3 standard (70 ppbv) (Fig. S4). The exceedance days decrease for
both PM2.5 and O3, especially in the eastern US. In 2010,
fewer than 5 days exceed the air quality standard for the majority of the US
(Fig. S4b, e). We also
calculate the population exposure exceedances by multiplying the population
(adults > 25 years old) by the number of air quality exceedance
days in each grid cell. The PM2.5 population exposure exceedances
have decreased from 5340 million people-days in 1990 to 1042 million people-days in 2010, and the
O3 population exposure exceedances has decreased from 4691 million
people-days in 1990 to 2236 million people-days in 2010 (Fig. S1). These
decreases in population exposure exceedances occur despite population growth
over this period.
Mortality burden trends and contributing factors
The mortality burdens associated with exposure to ambient PM2.5 in
the US steadily decreased by 54 %, from 123 700 (95 % confidence
interval considering the uncertainty in relative risk only,
70 800–178 100) deaths year-1 in 1990 to 58 600
(24 900–98 500) deaths year-1 in 2010 (Fig. 3). The leading cause
of PM2.5-related mortality is IHD, which decreases by 55 %,
from 96 500 (62 600–132 500) deaths year-1 in 1990 to 43 600
(21 500–68 700) deaths year-1, followed by LC, which has decreased
by 44 %, from 12 500 (2500–21 000) deaths year-1 in 1990 to
7000 (900–13 400) deaths year-1 in 2010 (Table S4). The
PM2.5 mortality burden per 100 000 adults is much higher in the
east than the west for both 1990 and 2010 (Fig. 4), due to the higher
PM2.5 concentrations (Fig. 1).
Trends in the total mortality burden (black) for
PM2.5 (a, as a total of ischemic heart disease
(IHD) + stroke (STROKE) + chronic obstructive
pulmonary disease (COPD) + lung cancer (LC)) and O3 (b,
chronic respiratory disease (RESP)), and mortality
burdens considering the air quality change only (blue), and with air quality
changes excluded (red). Units are deaths year-1. The
error bars are the 95 % CI for the total mortality burden (black).
The total mortality burdens in 2010 and the burdens in 2010 due to
changes since 1990 in each of three factors (concentration, baseline
mortality rates and population) and where the concentration change is
excluded, for PM2.5 and O3, and the relative changes
between 2010 and 1990. The relative changes are calculated as
(2010–1990) / 1990. The mortality burdens in the US for PM2.5
and O3 in 1990 are 123 700 deaths year-1 (70 800–178 100)
and 10 900 deaths year-1 (3700–17 500).
2010 (deaths year-1)
Relative changes
Mortality burden
58 600 (24 900–98 500)
-54 %
Concentration change only
78 900 (35 700–129 200)
-36 %
PM2.5
Mortality rate change only
68 300 (35 800–101 300)
-45 %
Population change only
173 500 (99 900–250 000)
40 %
Concentration change excluded
94 400 (50 300–140 000)
-24 %
Mortality burden
12 300 (4 100–19 800)
13 %
Concentration change only
8100 (2700–13 100)
-25 %
O3
Mortality rate change only
13 100 (4400–21 000)
20 %
Population change only
14 100 (4800–22 700)
30 %
Concentration change excluded
16 900 (5700–27 000)
55 %
Table 1 shows the mortality burdens for PM2.5 and O3 in 2010,
and also the burden changes since 1990 from different contributing factors.
From the table, we see that the PM2.5-related mortality burden in
2010 would have decreased by only 24 % (94 400 deaths year-1 in 2010,
95 %CI, 50 300–139 800) compared with that in 1990, if the PM2.5
concentrations had stayed constant over the period 1990–2010, due to
decreases in the baseline mortality rates for the specific causes of death
that PM2.5 influences (Fig. 3), especially IHD (Fig. 5), despite the
population increase. Therefore, the reduction in PM2.5 concentrations
from 1990 to 2010 significantly accelerates the decrease in the mortality
burden. The decreased PM2.5 concentration avoided roughly 35 800
(38 %) PM2.5-related deaths in 2010, compared to the case if
current air quality stays at level in 1990 (estimated as the 2010 mortality
burden minus the “concentration change excluded” case in 2010). The benefit
of the decreased PM2.5 concentration could also be estimated as the
“concentration change only” case in Fig. 3, yielding 78 900
(35 700–129 200) deaths year-1 in 2010, decreasing by 36 %
(-44800 deaths year-1) compared with 1990. The population increases from 1991 to
2010 would lead to increases in the PM2.5 mortality burden, but that
increase is smaller than the combined reduction from decreasing PM2.5
concentrations and baseline mortality rates (Figs. S5 and S6).
When separating the two 11-year periods, the PM2.5-related
mortality burden decreased by 45 % from 2000 to 2010 (decreasing trend of
-4400 deaths year-1), much higher than the 15 % decrease from
1990 to 2000 (decreasing trend of -2100 deaths year-1) (Table S5).
The detrended annual PM2.5-related mortality burden has a
coefficient of variation (CV, standard deviation divided by the average) of
4 %, mainly caused by inter-annual variation in PM2.5
concentrations (Table S6 and Fig. S6).
We also calculate burdens and trends for each state individually (Table 2).
The three states with the highest PM2.5 mortality burden in 1990
are New York (NY, 13 700 deaths year-1), California (CA,
9500 deaths year-1) and Pennsylvania (PA, 9200 deaths year-1);
and in 2010, NY (5100 deaths year-1), Texas (TX,
4200 deaths year-1) and Ohio (OH, 3900 deaths year-1). NY has
seen the largest benefits of mortality burden decreases
(-8500 deaths year-1), followed by CA (-6100 deaths year-1)
and PA (-5500 deaths year-1). For the relative mortality burden
changes, generally large percent decreases in PM2.5-related
mortality are seen in western, northern, and northeastern states (including
Nevada, Utah, Colorado, Montana, Maine and Vermont) (Fig. 6), because the
PM2.5 concentrations in 2010 are very low or even fall below the
low-concentration threshold in these states (Fig. 1), as confirmed by the
mortality burden changes from concentration changes alone (Table S7). For
other states in the eastern US with large relative mortality burden changes,
the contributing factors are different. For example, for Connecticut, the
relative mortality burden changes from the decrease in PM2.5
concentration are larger than that from the decrease in the baseline
mortality rates. However, for Massachusetts, NY and PA, the decreases in
baseline mortality rates have a slightly larger effect than that from the
decrease in PM2.5 concentration. For CA, the effects from the
decrease in baseline mortality rates and PM2.5 concentration are
comparable (Table S7).
The mortality burden for 48 US states and the District of
Columbia in 1990 and 2010, and the absolute changes from 1990 to 2010. Units
are deaths year-1.
PM2.5-related mortality
O3-related mortality
States
1990
2010
Diff
1990
2010
Diff
AL
2135
1166
-969
159
238
-12
AR
1127
752
-375
74
133
22
AZ
554
196
-358
125
329
138
CA
9515
3420
-6095
567
1272
359
CO
222
35
-187
115
230
64
CT
1795
458
-1337
93
129
-22
DC
250
157
-92
12
21
-6
DE
492
264
-227
26
54
14
FL
4688
2441
-2246
483
774
34
GA
3149
1954
-1195
221
413
51
IA
1500
756
-743
74
102
1
ID
174
120
-54
18
39
14
IL
7770
3547
-4223
280
500
38
IN
3821
2067
-1754
198
360
71
KS
1064
697
-367
84
147
26
KY
2420
1388
-1032
160
257
23
LA
1752
855
-898
109
195
4
MA
3417
1107
-2310
153
197
-57
MD
2893
1713
-1180
155
261
1
ME
347
5
-341
28
21
-19
MI
5894
2590
-3304
220
407
46
MN
1626
699
-927
61
107
16
MO
3135
1906
-1229
175
286
31
MS
1352
608
-743
75
124
6
MT
9
2
-7
12
19
1
NC
3321
1961
-1361
208
430
70
ND
75
23
-52
8
12
-1
NE
535
257
-278
55
81
7
NH
453
73
-380
24
25
-13
NJ
5332
2196
-3137
223
404
28
NM
245
180
-65
37
109
42
NV
10
0
-10
52
138
60
NY
13 712
5239
-8473
406
613
-88
OH
7876
3932
-3944
400
690
103
OK
1499
1058
-441
120
248
77
OR
633
219
-413
39
42
-15
PA
9238
3727
-5511
393
584
-70
RI
630
172
-457
32
44
-5
SC
1673
974
-699
109
218
30
SD
140
68
-72
14
23
3
TN
3097
1895
-1202
199
317
13
TX
6499
4178
-2321
417
896
228
UT
107
10
-96
25
72
29
VA
2806
1592
-1214
183
336
29
VT
196
14
-182
11
8
-9
WA
917
394
-522
71
75
-27
WI
2479
977
-1503
75
148
35
WV
1161
534
-627
84
122
-9
WY
1.2
0.4
-0.8
12
25
8
The mortality burdens associated with PM2.5 (a, b), O3 (d, e) in 1990 (a, c) and
2010 (b, d), and the differences (2010 minus 1990) (c, f) for each
36 km × 36 km grid cell. Units are deaths year-1
per 100 000 adults (above 25 years old).
The mortality burden associated with exposure to O3 from RESP has
increased by 13 %, from 10 900 (3700–17 500) deaths year-1 in
1990 to 12 300 (4100–19 800) deaths year-1 in 2010 (Fig. 3). The
O3 mortality burden per 100 000 adults is highest in the midwest and
southwest (Fig. 4). The O3-related mortality burden in 2010 would
have increased by 55 % (10 600 deaths year-1 in 2010, 95 %
CI, 3600–17 100) compared with that in 1990 if the O3 concentration
had stayed constant over the period 1990–2010 (Fig. 3), due to increases in
both population and baseline mortality rates (Fig. S5). The decreased
O3 concentration would have avoided roughly 4600 (27 %)
O3-related deaths in 2010, compared to the case if ozone
concentrations stay at level in 1990 (estimated as the 2010 mortality burden
minus the “concentration change excluded” case in 2010). The benefit of the
decreased O3 concentration could also be estimated as the
“concentration change only” case in Fig. 3, yielding 8100
(2700–13 100) deaths year-1 in 2010, decreasing by 25 %
(-2800 deaths year-1) compared with 1990. The change in O3
generally reduces the mortality burden relative to 1990 with some
inter-annual variation (Fig. S6) due to meteorology and wildfires (Porter et
al., 2017), while the increases in population and baseline mortality rates
generally increase the mortality burden, with a larger contribution from the
population change (Fig. S6).
When separating the O3 mortality trends into 2 decades, we find that
the burdens decrease slightly (-70 deaths year-1) from 2000 to 2010,
compared with the increasing trend from 1990 to 2000
(240 deaths year-1) (Table S5). The increasing trend in the first
decade is caused by the combined effect of increases in baseline mortality
rates and population, while the decreasing trend in the second decade is
dominated by decreases in O3 concentration (Fig. S6). The
inter-annual variability for the detrended annual O3 mortality burden
from 1990 to 2010 (CV of 12 %) is larger than PM2.5 (CV of
4 %), caused mainly by variations in O3 concentrations from 1990
to 2010 (Table S6).
The three states with the highest O3 mortality burden in 1990 are CA
(910 deaths year-1), Florida (FL, 740 deaths year-1) and NY
(700 deaths year-1); and in 2010, CA (1270 deaths year-1), TX
(900 deaths year-1) and FL (770 deaths year-1) (Table 2). CA
has seen the largest O3 mortality burden increases
(360 deaths year-1), followed by TX (230 deaths year-1) and
Arizona (AZ, 140 deaths year-1), with the greatest decrease in NY
(-90 deaths year-1). For the relative mortality burden changes,
large percent decreases in O3-related mortality are seen in the
northwestern (Washington and Oregon) and northeastern US (Fig. 6), mainly
caused by significant O3 decreases (Table S7), while the greatest
percent increases occur in the southwestern US driven mainly by large
population increases, and also the baseline mortality rate increases.
Previous health impact assessments have used national baseline mortality
rates (Cohen et al., 2017; Silva et al., 2016a, b, etc.), but baseline
mortality rates can vary strongly within individual counties (Fig. 5;
Dwyer-Lindgren et al., 2016). We performed sensitivity analyses by applying
the national average baseline mortality rates for each disease to every
county in the mortality burden calculations. We find that the PM2.5
mortality burden calculated from the national average baseline mortality
rates is lower than those calculated from the county-level baseline mortality
rates, ranging among individual years from -2.2 % to -1.3 %
(Table S8). For the O3 mortality burden, the differences between
using the national average baseline mortality rates and our best estimates
range from -1.1 % to 2.0 % (Table S8). However, using the national
average baseline mortality rates fails to capture regional mortality burden
hotspots for both PM2.5 and O3 (Figs. S7–S8),
demonstrating the value of using county-level baseline mortality rates where
possible.
The baseline mortality rates for specific causes of death related to
PM2.5, including chronic obstructive pulmonary
disease (a), lung cancer (b), ischemic heart
disease (c) and stroke (d), and respiratory diseases
related to O3 (e). The bottom whiskers, bottom border,
middle line, top border and top whiskers of the boxes indicate the 5th, 25th,
50th, 75th, and 95th percentiles, respectively, across all counties; the red
circles are the national average rate. Baseline mortality rates are shown for
1990–1998 after they are corrected to ensure comparability between ICD9 and
ICD10 codes. The units on the y axis are per 100 000 people.
Comparison with previous studies
The mortality burden associated with PM2.5 calculated in our study
generally aligns with several previous findings (Fig. 7; also Table S9). Our
PM2.5 mortality burden is higher than that reported by Cohen et
al. (2017) in 1990 (17 % higher) and 1995 (4 % higher), and lower in
2000 (-0.5 %), 2005 (-17 %) and 2010 (-30 %) (Fig. 7). The
overestimation of PM2.5 mortality burdens in the early 2000s are
likely due to the higher population-weighted PM2.5 concentration
simulated by WRF-CMAQ (Fig. 2), compared with Cohen et al. (2017), in which
they estimated the PM2.5 concentration based on data fusion of air
quality model outputs, satellite retrievals and ground observations. The
lower mortality burdens in the second decade (from 2000 to 2010) in our study
likely reflect that Cohen et al. (2017) included hemorrhagic stroke and lower
respiratory infections in the PM2.5-related mortality burden, in
addition to COPD, LC, IHD and STROKE, and used an updated integrated
exposure–response function. While the absolute value is similar, our results
show a stronger decreasing trend (-3000 deaths year-1) than Cohen et
al. (2017) (-1000 deaths year-1), which may result from the
overestimation of PM2.5 decreasing trends in our model relative to
ground observations (Gan et al., 2016). The PM2.5 mortality burdens
estimated in our study are much lower than those from Fann et al. (2017), but
the temporal patterns are similar, mainly because Fann et al. (2017)
estimated the total all-cause mortality with a different HIF.
Relative mortality burden changes from 1990 to 2010 for
the 48 states and the District of Columbia for PM2.5 (a) and O3 (b). The relative changes are
calculated as (2010–1990)/1990 × 100 %. Note the different color
scales for the two plots. The values for the District of Columbia are
-37 % for PM2.5 and -23 % for
O3.
Comparisons of the US mortality burdens attributed to
PM2.5 (a) and O3 (b) in this study, with
Cohen et al. (2017), Fann et al. (2012a, 2017), Punger and West (2013), and
Giannadaki et al. (2016). The black line for O3 is the recalculated O3
mortality burden from the COPD, and the black dashed line is the recalculated
O3 mortality burden from RESP using the pre-industrial O3
concentration as the counterfactual risk exposure factor. The error bars show
the 95 % CI from the RRs, shown for this study and Cohen et al. (2017).
To compare with Cohen et al. (2017), who reported the O3 mortality
burden from the COPD, which is a subset of RESP, we recalculate the
O3 mortality burden from the COPD (Table S4). The newly calculated
O3 mortality burden from the COPD is generally lower than the
estimate of Cohen et al. (2017) by 8 %–30 % (Fig. 7). This could be
caused by the fact that for the O3 changes, we use the summertime
(April to September) average of the 1 h daily maximum, while Cohen et
al. (2017) used the 3-month average, which will be higher. The temporal trend
for the O3 mortality burdens from our study is similar to that from
Cohen et al. (2017), except that the burden decreases after 2005 in our study
but increases in Cohen et al. (2017). The O3 mortality burden from
the RESP disease in 2005 estimated from our study is much lower than two
previous studies (Fann et al., 2012a; Punger and West, 2013; Table S10). As
discussed in the methods, the lower US background O3 concentration
used in these two studies (22 ppb in the eastern US and 30 ppb in the
western US) could lead to a higher O3 mortality burden. We then did
sensitivity analysis by using the pre-industrial O3 concentration
simulated by an ensemble of model outputs from the Atmospheric Chemistry and
Climate Model Intercomparison Project (Lamarque et al., 2013; see Fig. S9 and
Sect. 2 in the Supplement) as the counterfactual risk exposure factor, and
recalculated the O3 mortality burden with RESP. The new calculated
O3 mortality burdens are estimated to be 64 %–100 % higher
than the current estimation from RESP using the low-concentration threshold
(Table S10). In Fig. 7, we see that the new estimated O3 mortality
burden from RESP in 2005 (dashed line) is now comparable with the two
previous studies.