A STUDY OF SHORT-TERM EFFECTS OF AMBIENT AIR POLLUTION ON PUBLIC HEALTH

 

5.0 RESULTS

5.1

Summary Statistics of Levels of Air Pollutants and Meteorological Variables

Tables 5 and 5a present the summary statistics of various measures of daily concentrations of individual pollutants and meteorological variables for the study period (1994 - 95, and the first half of 1996). The mean daily values are well below the Air Quality Objectives set by the Environmental Protection Department. Mean daily levels of air pollutants are generally higher in the first half-year of 1996 than in 1994 - 95, with an increase of 3% for RSP, 5% for NO2, 14% for SO2 and 36% for O3.

Table 5: Summary statistics of daily pollutant concentrations and meteorological variables (1994-95)
 
  Mean SD Min P25 Median P75 Max
NO2 max  84.97 32.61 25.13 59.53 81.44 108.41 194.40
NO2 mean  53.67 18.83 16.41 39.93 51.39 66.50 122.44
O3 max (24hr)  52.90 29.01 1.50 26.50 51.50 75.00 222.00
O3 mean (24hr)  26.66 18.22 0.20 10.67 20.65 39.30 88.07
O3 mean (8hr)  28.69 20.70 0 11.73 24.13 43.72 207.00
SO2 max  47.93 33.32 4.67 23.93 35.95 63.20 226.50
SO2 mean  20.18 11.35 2.74 12.43 17.10 25.05 68.49
RSP max  81.90 39.06 23.17 51.01 71.80 106.16 320.42
RSP mean  50.12 24.33 14.77 30.64 44.57 65.44  159.73
Temp mean  22.72 5.11 9.34 18.40 24.00 27.15 30.96
Temp min  20.32 5.29 6.61 15.97 21.86 24.90 28.35
Hum max  89.77 7.40 47.29 87.00 91.85 94.67  96.33
Hum mean  77.92 10.85 32.33 73.41 80.14 85.00 96.33

 

Table 5a: Summary statistics of daily pollutant concentrations and meteorologicalvariables (1996, first half)
 
  Mean SD Min P25 Median P75 Max
NO2 mean  56.22 19.14 19.95 45.29 54.67 67.54 121.59
O3 mean (8hr)  39.11 34.72 4.06 16.16 28.78 51.72 209.97
SO2 mean  17.72 11.72 2.62 9.25 14.89 23.56 90.25
RSP mean  51.65 27.24 14.14 30.50 44.37 67.54 161.8
Temp mean  20.79 5.71 5.37 17.21 20.55 25.30 29.80
Temp max  23.52 5.81 6.60 20.07 23.45 27.70 33.20
Temp min  18.59 5.94 4.18 14.30 18.06 23.80 28.30
Hum mean  78.40 9.46 46.00 73.00 79.00 85.00 96.00

The three-monthly mean of the daily concentrations of air pollutants are shown in Tables 6 to 9. Levels are generally higher in colder seasons for NO2 and RSP. A distinct pattern is observed for O3, when levels in the last quarter of each year are about twice those of the other quarters. The levels of SO2, after reaching a peak in the third quarter of 1994, show a steady decline throughout 1995 but increase slightly in 1996.

 

Table 6 : 3-monthly mean, S.D. and range of daily SO2, 1994 - 1996 *
 
Time period No. of observations Mean S.D. Min Max
Jan-Mar 94 90 20.23 10.22 2.75 48.93
Apr-Jun 94 91 24.42 11.92 6.22 54.66
Jul-Sep 94 92 25.63 15.53 6.69 68.49
Oct-Dec 94 92 18.15 6.53 6.06 46.65
Jan-Mar 95 90 20.83 11.65 8.57 64.14
Apr-Jun 95 91 18.69 12.13 4.80 53.13
Jul-Sep 95 90 17.60 10.44 4.67 51.98
Oct-Dec 95 92 15.90 6.21 2.74 33.18
1994 - 1995 728 20.18 11.35 2.74 68.49
Jan-Mar 96 91 17.32 11.10 2.62 55.46
Apr-Jun 96 91 18.11 12.35 5.26 90.25

* first half-year

 

Table 7: 3-monthly mean, S.D. and range of daily NO2, 1994 -1996 *
 
Time period No. of observations Mean S.D. Min Max
Jan-Mar 94 90 63.52 17.29 32.93 111.44
Apr-Jun 94 91 44.94 16.69 19.43 97.11
Jul-Sep 94 92 45.70 19.71 18.94 105.11
Oct-Dec 94 92 60.05 15.50 38.43 111.05
Jan-Mar 95 90 61.94 16.64 35.04 122.44
Apr-Jun 95 91 46.29 16.15 21.68 101.49
Jul-Sep 95 92 43.03 15.21 16.41 91.98
Oct-Dec 95 92 64.14 16.10 19.82 98.09
1994 - 1995 730 53.67 18.83 16.41 122.44
Jan-Mar 96 91 63.60 18.05 29.37 121.59
Apr-Jun 96 91 48.85 17.36 19.95 118.39

* first half-year

 

Table 8 : 3-monthly mean, S.D. and range of daily RSP, 1994 -1996 *
 
Time period No. of observations Mean S.D. Min Max
Jan-Mar 94 84 60.69 26.20 16.91 142.82
Apr-Jun 94 91 35.91 13.60 16.27 90.45
Jul-Sep 94 91 40.72 21.23 17.13 101.47
Oct-Dec 94 92 56.41 20.82 14.96 105.93
Jan-Mar 95 90 58.16 22.69 18.62 159.73
Apr-Jun 95 91 41.33 18.18 19.45 112.20
Jul-Sep 95 92 37.06 17.29 14.77 94.91
Oct-Dec 95 92 71.38 26.01 19.59 142.37
1994 - 1995 723 50.12 24.33 14.77 159.73
Jan-Mar 96 91 60.37 27.49 14.14 119.98
Apr-Jun 96 91 42.92 24.15 19.41 161.81

* first half-year

 

Table 9: 3-monthly mean, S.D. and range of daily O3, 1994 -1996 *
 
Time period

No. of observations

Mean S.D. Min Max
Jan-Mar 94 90 27.16 18.77 1.94 94.31
Apr-Jun 94 90 22.06 18.30 2.23 81.31
Jul-Sep 94 92 22.80 21.21 1.38 129.94
Oct-Dec 94 87 40.48 22.42 0 88.81
Jan-Mar 95 90 24.51 18.22 0.31 65.54
Apr-Jun 95 91 22.03 15.09 0 61.38
Jul-Sep 95 90 24.60 18.31 0.06 76.88
Oct-Dec 95 92 46.08 17.97 4.75 100.06
1994 - 1995 722 28.69 20.70 0 129.94
Jan-Mar 96 89 36.98 27.78 4.56 172.96
Apr-Jun 96 91 41.19 40.42 4.06 209.97

first half-year

   
5.2

Time Trends of Air Pollutants

The time trends of individual air pollutants for 1994 - 95 and 1996 (first half-year) are shown in Figures 1 - 4 and 1a - 4a respectively. For NO2 and RSP, a distinct cyclical pattern with a long wavelength is observed, and higher levels in winter. For O3 and SO2, shorter wavelengths are seen, with peaks in early spring and late autumn for the former and mid-summer for the latter. There is no obvious increasing trend for all four pollutants during the 1994 - 94 period. Levels of O3 are generally higher in the first half of 1996 compared to 1994 - 95.


View Graphs:

  1. Figure 1: Daily mean levels of nitrogen dioxide from January 1, 1994 to December 31,1995 (Average of seven monitoring stations)
  2. Figure 1a: Daily mean levels of nitrogen dioxide from January 1 to June 30, 1996 (Average of seven monitoring stations)
  3. Figure 2: Daily mean levels of respirable suspended particulars from January 1, 1994 to December 31, 1995 (Average of five monitoring station)
  4. Figure 2a: Daily mean levels of respirable suspended particulars from January 1 to June 30, 1996 (Average of five monitoring station)
  5. Figure 3: 8-hour mean levels of ozone from January 1, 1994 to December 31, 1995 (Average of two station)
  6. Figure 3a: 8-hour mean levels of ozone from January 1 to June 30, 1996 (Average of two station)
  7. Figure 4: Daily mean levels of sulphur dioxide from January 1,1994 to December 31, 1995 (Average of six station)
  8. Figure 4a: Daily mean levels of sulphur dioxide from January 1 to June 30, 1996 (Average of six station)

5.3

Summary Statistics of Hospital Admissions

Summary statistics of daily counts of hospital admissions for respiratory and circulatory diseases in 1994 and 1995, and in the first half of 1996 are shown in Tables 10 and 10a respectively. The daily mean numbers of admissions in 1996 (first half-year) was generally higher than those in 1994 to 1995 (by 32% for respiratory diseases, 20% for cardiovascular diseases and 27% for both combined).

Table 10: Summary statistics of daily hospital admissions for respiratory and cardiovascular diseases (1994-95)
 
  Mean SD Min P25 Median P75 Max
Total *  235.83 39.05 151 207 233 259 383
Respiratory  134.44 24.89 84 116.75 131 150 232
Cardiovascular  101.39 20.57 54 87 101 116 177
Asthma  22.22 7.84 6 16 21 27 51
AMI**  4.05 2.61 0 2 4 6 14

* Total = Respiratory diseases + Cardiovascular diseases

** Acute myocardial infarction

 

Table 10a: Summary statistics of daily hospital admissions for respiratory and cardiovascular diseases (1996, first half-year)
 
  Mean SD Min P25 Median P75 Max
Total *  298.53 49.44 192 261.75 295.5 331.25 427
Respiratory  177.14 30.54 108 152.75 176 201 251
Cardiovascular  121.39 28.76 61 101 122 140 200

* Total = Respiratory diseases + Cardiovascular diseases

The total numbers of admissions for 1994-95 by age and type are shown in Table 10b. A total of 172,157 hospital admissions for respiratory and cardiovascular (circulatory) diseases were recorded in the selected hospitals during the period 1994-95 and used in the Poisson regression model while 171,829 were used (i.e., excluding 328 admissions with no record of age) in the analysis by age group

 

Table 10b: Number of admissions due to respiratory and cardiovascular diseases by hospital in 1994-95

Age group

  0 - 4 5 - 14 15 - 64 65+ unknown total
Cardiovascular diseases 420 438 22,871 50,116 169 74,014
Respiratory diseases 30,263 9,864 20,229 37,628 159 98,143
Both diseases 30,683 10,302 43,100 87,744 328 172,157
   
5.4

Time Trends of Hospital Admissions


Time trends of hospital admissions for respiratory and cardiovascular diseases for the period 1994 - 1995 and the first half of 1996 are shown in Figures 5 - 7 and 5a - 7a respectively. An increasing trend throughout the overall study period is evident, with a cyclical pattern more obvious in 1994. This was contributed largely by respiratory diseases, with an approximately three-month cyclical pattern, while hospital admissions for cardiovascular diseases displayed a sinusoidal pattern with a longer period. For the first half of 1996, the sinusoidal pattern was less obvious (Fig. 5a - 7a).


View Graphs:

  1. Figure 5: Daily hospital admissions (respiratory and cardiovascular diseases) from January 1, 1994 to December 31, 1995
  2. Figure 5a: Daily hospital admissions (respiratory and cardiovascular diseases) from January 1 to June 30, 1996
  3. Figure 6: Daily hospital admissions (respiratory diseases) from January 1, 1994 to December 31, 1995
  4. Figure 6a: Daily hospital admissions (respiratory diseases) from January 1 to June 30, 1996
  5. Figure 7: Daily hospital admissions (cardiovascular diseases) from January 1, 1994 to December 31, 1995
  6. Figure 7a: Daily hospital admissions (cardiovascular diseases) from January 1 to June 30, 1996

5.5

Correlation between Individual Air Pollutants, and between Air Pollutants and Hospital Admissions

The correlations between individual air pollutants from 1994 to 1995 and for each season are shown in Table 11. Overall, the correlation between NO2 and RSP was good, while those between RSP and O3, NO2 and SO2, and NO2 and O3 were fair. In general, the correlation between pollutants was higher in summer, except between SO2, and NO2, where the correlation coefficient was the highest in winter. The high degree of collinearity between air pollutants, in particular, NO2 and RSP cautions against the use of a multiple pollutant model in the analysis.

Table 11: Pearson correlation coefficients between mean daily concentrations of pollutants (1994-95)
 
    Summer
(Jun-Aug)
Autumn
(Sep-Nov)
Winter
(Dec-Feb)
Total
SO2 & NO 20.20** 0.71*** 0.57*** 0.82*** 0.45***
SO2 & RSP  0.13 0.69*** 0.41*** 0.58*** 0.31***
SO2 & O3 (8 hr)  -0.38*** 0.22** -0.02 -0.08 -0.12***
NO2 & RSP 0.70*** 0.86*** 0.77*** 0.71*** 0.79***
NO2 & O3 (8 hr)  0.26*** 0.50*** 0.45*** 0.27*** 0.44***
RSP & O3 (8 hr)  0.22** 0.59*** 0.54*** 0.41***  0.51***

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

The correlations between individual air pollutants and hospital admissions for respiratory and cardiovascular diseases from 1994 to 1995 are shown in Table 12. In general, the (unadjusted) correlation was poor. Of all four air pollutants, mean NO2 and RSP showed positive and significant correlations with total hospital admissions as well as for respiratory and cardiovascular diseases. Mean and minimum daily temperatures were negatively correlated with hospital admissions.

 

Table 12: Correlation coefficients between air pollutants and hospital admissions (1994-95)
 
Total Cardiovascular Respiratory
SO2 mean  0.0389 0.0561 0.0146
NO2 mean 0.2039*** 0.2148*** 0.1424***
O3 mean (24h ) 0.0429 0.0225 0.0488
O3 mean (8h)  -0.0112 -0.0259 0.0039
RSP mean  0.1797*** 0.1757*** 0.1367***
Temp mean  -0.1516*** -0.1975*** -0.0747*
Temp min  -0.1646*** -0.2084*** -0.0860*

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

   
5.6

Parameter Estimates using Multiple Linear Regression Model

Using data on total hospital admissions (respiratory and cardiovascular diseases), a multiple linear regression model was fitted to estimate the partial regression coefficients of variables in the core model (without air pollutants) and the coefficient of determination using SAS Program Version 6.1 (SAS Institute Inc., 1996). R2 (coefficient of determination) was 0.6609 indicating that the confounding variables alone, accounted for about two third of the variations in daily hospital admissions (Table 13). When the daily hospital deaths due to respiratory and cardiovascular diseases combined were fitted into the model, the R2 was 0.3098, indicating a poorer fit compared with the hospital admission data.

 
Table 13: Estimates of partial regression coefficients of variables in the core model using multiple linear regression

Dependent Variable : Logarithm* of total (respiratory + cardiovascular) admissions

Analysis of Variance
Source  DF Sum of Squares Mean Square F Value Prob F
Model  21 12.98416 0.61829 65.699 0.0001
Error  708 6.66303 0.00941    
C Total  729 19.64720      
Root MSE  0.09701 R-square 0.6609    
Dep Mean  5.44963 Adj R-sq 0.6508    
C.V 1.78013        
 
Parameter Estimates
Variable ** DF

Parameter Estimate

Standard Error 

T for H0: Parameter=0

Prob |T|
Intercept  1 5.093112 0.05145 98.991 0.0001
1 0.000498 0.00012 4.302 0.0001
t 2 1 -0.000000381 0.0000001 -4.040 0.0001
Year 1 0.086919 0.03470 2.505 0.0125
CS1 1 0.047362 0.01184 3.999 0.0001
CS2 1 -0.015222 0.00516 -2.948 0.0033
CS3 1 -0.023926 0.00517 -4.631 0.0001
CS4 1 0.007174 0.00511 1.403 0.1611 (NS) #
S1 1 0.086865 0.01319 6.583 0.0001
S2 1 0.024817 0.00802 3.093 0.0021
S3 1 -0.044763 0.00627  -7.136 0.0001
S4 1 -0.009669 0.00578 -1.673 0.0948 (NS)
I1* 1 0.235993 0.01344 17.555 0.0001
I2 1 0.158011 0.01343 11.762 0.0001
I3 1 0.194218 0.01345 14.440 0.0001
I4 1 0.126288 0.01345 9.391 0.0001
I5 1 0.094318 0.01344 7.018 0.0001
I6 1 0.020710 0.01340 1.546 0.1226 (NS)
Holiday 1 1 -0.139361 0.01631 -8.545 0.0001
Holiday 2 1 0.055351 0.02345 2.360 0.0185
Temp mean 1 0.005759 0.00187 3.077 0.0022
Humidity mean 1 -0.000567 0.00042 -1.350 0.1773 (NS)

* Logarithmic transformation was used to "normalize" the data.
** Caption of variables:

t: time trend S1-4: sine terms for seasonality
Year: year-effect indicator CS1-4: cosine terms for seasonality
Holiday 1: Public Holiday Holiday 2: Day after Public Holiday
I1 - I6: days of the week (Monday - Saturday)  

# NS: Not statistically significant

   
5.7

Parameter Estimates using Poisson Regression Model

The logistic procedure of the SAS 6.1 Programme (SAS Institute Inc., 1996) was used to estimate the parameters and their risk ratios in the core model (Table 14). The variables which were statistically significant in the Poisson core model were almost identical to those in the multiple linear regression model.

Table 14: Parameter estimates and risk ratios of variables in core model (Poisson regression)

Response Profile

Ordered value Binary outcome Count
1 Event 172,157
2 No event 4.3798E9


Deviance and Pearson goodness-of-fit Statistics

Criterion DF Value Value/DF P Chi-square
Deviance 708 1535.9 2.1693 ^ 0.0001
Pearson 708 1535.2 2.1684 0.0001
Number of events / trials observations: 730


Model fitting and testing global null hypothesis big_b.gif= 0

Criterion Intercept only Intercept & covariates  Chi-square for covariates
AIC* 3837082.9 3834011.1 -
SC # 3837013.1 3834455.5 3113.813 with 21 DF (p = 0.0001)
-2 log L $ 3837080.9 3833967.1 3111.311 with 21 DF (p = 0.0001)

* Akaike Information Criterion
# Schwartz Criterion
$ -2 log Likelihood

The -2 log Likelihood has a chi-square distribution under the null hypothesis (that all the explanatory variables in the model are zero), and a p value is given for this statistic. The AIC and SC statistics give two different ways of adjusting the -2 log Likelihood statistic for the number of terms in the model and the number of observations used. Lower values of the statistic indicate a better fitting model.

^ This value, ø, is an estimate of overdispersion parameter.

Table 14 :
(continued)
Parameter estimates and risk ratios of variables in Poisson regression core model

Analysis of maximum likelihood estimates

Variable* DF Parameter estimate Standard error Wald Chi-square p c2 Standardized estimate Odds ratio
Intercept 1 -10.5163 0.0349 90738.9572 0.0001 - -
t 1 0.000495 0.00008 38.3764 0.0001 0.057487 1.000
t 2 1 -3.62E-7 6.35E-8 32.4615 0.0001 -0.031733 1.000
Year  0.0859 0.0237  13.1398  0.0003 0.023686 1.090
 CS1 1 0.0508 0.00799 40.5041 0.0001 0.019813 1.052
 CS2 1 -0.0149 0.00347 18.4443 0.0001 -0.005807 0.985
 CS3 1 -0.0234 0.00347 45.6064 0.0001 -0.009141   0.977
CS4 1 0.00508 0.00344 2.1837 0.1395 0.001982 1.005
S1 1 0.0906 0.00899 101.5405 0.0001 0.035323 1.095
S2  1 0.0253 0.00541 21.7865 0.0001 0.009852 1.026
S3 1 -0.0445 0.00422 111.2212 0.0001 -0.017359  0.956
S4 1 -0.0106 0.00388 7.4748 0.0063 -0.004138 0.989
I1 * 1 0.2372 0.00908 682.5627 0.0001 0.045711 1.268
I2 1 0.1563 0.00923 286.8563 0.0001 0.030119 1.169
I3 1 0.1942 0.00915 450.2165 0.0001 0.037415 1.214
I4 1 0.1278 0.00929 189.1263 0.0001 0.024621 1.136
I5 1 0.0945 0.00937 101.8439 0.0001 0.018218 1.099
I6 1 0.0185 0.00953 3.7720 0.0521 0.003580 1.019
Holiday 1 1 -0.1406 0.0117 144.7948 0.0001 -0.017434 0.869
Holiday 2 1 0.0508 0.0153 10.9657 0.0009 0.004341 1.052
Temp.mean 1 0.00619 0.00125 24.4479 0.0001 0.017419 1.006
Humid.mean 1 -0.00063 0.00028 5.0089 0.0252 -0.003742 0.999

* Caption of variables:
t: time trend
Year: year-effect indicator
I1 - I6: days of the week (Monday to Saturday)
S1-4: sine terms for seasonality
CS1-4: cosine terms for seasonality
Holiday 1: Public Holiday
Holiday 2: Day after Public Holiday

   
5.8

The fitted Core Model and the Residual Plot

Figures 8 - 9 show the predicted number of daily admissions based on the multiple linear regression core model (without air pollutants), and the residuals of the model. After fitting the core model, the residuals appear to be more randomly dispersed and no obvious trend can be discerned. Figures 10 - 17 show the deviance residuals, by levels of pollutants and time (days) when each of the four pollutants is fitted into the Poisson regression model (Williams' method). The residual plots by pollutant levels and by time appear dispersed with no recognizable pattern,


View Graphs:

  1. Figure 8: Predicted daily total number of hospital admissions (respiratory and cardiovascular diseases) from January, 1994 to December 31, 1995
  2. Figure 9: Time plot of residuals of daily total number of admissions from a model including all potential confounding variables
  3. Figure 10: Plot of deviance residual of total (respiratory + cardiovascular) hospital admissions against NO2 LEVELS (ug.m-1) (NO2 fitted in model)
  4. Figure 11: Time series plot of deviance residual of total (respiratory + Cardiovascular) hospital admissions (NO2 fitted in model)
  5. Figure 12: Plot of deviance residual of total (respiratory + cardiovascular) hospital admissions against SO2 levels (ug.m-1) (SO2 fitted in model)
  6. Figure 13: Time series plot of deviance residual of total (respiratory + cardiovascular) hospital admissions SO2 fitted in model)
  7. Figure 14: Plot of deviance residual of total (respiratory + cardiovascular) hospital admissions against RSP levels (ug.m-1) (RSP fitted in model)
  8. Figure 15: Time series plot of deviance residual of total (respiratory + cardiovascular) hospital admissions (RSP fitted in model)
  9. Figure 16: Plot of deviance residual of total (respiratory + cardiovascular) hospital admissions against O3 levels (ug.m-1) (O3 fitted in model)
  10. Figure 17: Time series plot of deviance residual of total (respiratory + cardiovascular) hospital admissions (O3 fitted in model)

5.9 Risk Estimates using the Single Pollutant Model
   
 
5.9.1

Relative Risk Estimates of Hospital Admissions

Using the single pollutant model, parameters of individual pollutants (lag day 0 - 3, cumulative lag from day 0 up to day 3 for NO2, SO2, RSP, and up to day 5 for O3) were fitted into a logistic (Poisson) regression model with Williams' correction for overdispersion (SAS Institute Inc., 1996). For each pollutant, the parameter with the highest chi square value (i.e., the best explanatory value) was selected. The relative risks (RR) of hospital admissions for respiratory and cardiovascular diseases associated with every 100 ug.m-3 increase in the level of air pollutants are shown in Table 15. Significant risks of total (respiratory and cardiovascular diseases) hospital admissions and admissions for respiratory diseases were found for all four air pollutants (NO2, SO2, RSP and O3). For total admissions (respiratory and cardiovascular diseases), RR was highest for O3 lag0-5 (1.29), followed closely by NO2 lag0-1 (1.24). RR for RSP lag0-3 wassimilar, at 1.21, while that for SO2 lag0 was lowest, at 1.14. For respiratory diseases, RR were highest and almost identical for O3 lag0-3 and NO2 lag0-3 (at 1.40 and 1.39 respectively), followed by RSP (at 1.33) and SO2 (at 1.13). For bronchial asthma, RR for NO2 and O3 were even higher, and highly significant, at 1.61 and 1.51 respectively, followed by RSP (1.34) and SO2 (1.25). RR for circulatory diseases were lower for SO2, NO2 and O3 (at 1.18, 1.14 and 1.13 respectively), but only marginally significant for RSP. For acute myocardial infarction, RR were insignificant for NO2, SO2 and RSP, but less than unity for O3, (RR = 0.78, with marginal significance).

Relative risks (per 100 ug.m-3 increase) of air pollutants by age groups are shown in Table 15a. RRs of respiratory diseases admissions for NO2 and RSP were higher among the 0-4 years age group compared to the other groups, which had similar RRs. For O3 and SO2, the elderlies (65 and over) had significant and much higher RRs than their younger counterparts, of which the RRs were comparable and significant for O3 but insignificant for SO2. For circulatory illnesses, RRs for all four pollutants were significantly higher than unity in the elderlies but insignificant in the 5-64 age group. For total admissions (respiratory and circulatory), the RRs for all four pollutants were significant for the 5-64 and 65 and above, with higher RRs among the elderlies except for RSP. Because of the small number, RRs for circulatory diseases have not been imputed for the 0-4 years age group.

Table 15: Relative risk and 95% confidence intervals for 100 ug/m3 increase in the levels of air pollutant for total hospital admissions, respiratory and cardiovascular diseases (1994-95). Results for the best lags are shown.
 

Relative risk (95% confidence intervals)

Pollutant

Total admissions

Respiratory + circulatory

Respiratory Circulatory Asthma AMI
NO2

Lag0-1 1.24***

(1.18-1.30)

Lag0-3 1.40***

(1.30-1.51)

Lag0-1 1.14***

(1.07-1.22)

Lag0-3 1.61***

(1.39-1.89)

Lag0 0.83

(0.65-1.07)

SO2

Lag0 1.14***

(1.07-1.22)

Lag0 1.13**

(1.04-1.24)

Lag0-1 1.18***

(1.07-1.30)

Lag0 1.25*

(1.04-1.24)

Lag2 0.75

(0.52-1.07)

RSP

Lag0-3 1.21***

(1.15-1.27)

Lag0-3 1.33***

(1.25-1.42)

Lag0-2 1.06*

(1.00-1.12)

Lag0-3 1.34***

(1.17-1.53)

Lag2 0.84

(0.70-1.02)

O3(8h)

Lag0-5 1.29***

(1.21-1.37)

Lag0-3 1.39***

(1.29-1.50)

Lag0-5 1.13**

(1.04-1.23)

Lag0-2 1.51***

(1.30-1.76)

Lag4 0.78*

(0.62-0.96)

Lag0-1 to Lag0-5 indicates the cumulative lag from day 0 to day 1, Éup to day 5 respectively.

* 0.05 > p > 0.01
** 0.01 > p > 0.001
*** p < 0.001

 

Table 15a: Relative risks and 95% CI for 100 ug/m3 increase in the level of air pollutants by age group
 
Age group Total Admissions Respiratory Admissions Circulatory Admissions
NO2 Lag 0-1 Lag 0-3 Lag 0-1
0-4 - 1.42 *** (1.25, 1.60) -
5-64 1.20 *** (1.12, 1.30) 1.40 *** (1.24, 1.57) 1.08 (0.97, 1.21)
65+ 1.24 *** (1.17, 1.32) 1.40 *** (1.26, 1.56) 1.17 *** (1.10, 1.26)
Overall 1.24 *** (1.18, 1.30) 1.40 *** (1.30, 1.51) 1.14 *** (1.07, 1.22)
SO2 Lag 0 Lag 0 Lag 0-1
0-4 - 1.00 (0.87, 1.14) -
5-64 1.06 (0.96, 1.17) 1.10 (0.97, 1.26) 1.05 (0.89, 1.24)
65+ 1.25 *** (1.15, 1.35) 1.28 *** (1.14, 1.44) 1.25 *** (1.12, 1.39)
Overall 1.14 *** (1.07, 1.22) 1.13 ** (1.04, 1.24) 1.18 *** (1.07, 1.30)
RSP Lag 0-3 Lag 0-3 Lag 0-2
0-4 - 1.42 *** (1.28, 1.57) -
5-64 1.19 *** (1.10, 1.28) 1.30 *** (1.18, 1.43) 1.05 (0.95, 1.16)
65+ 1.17 *** (1.10, 1.24) 1.29 *** (1.18, 1.41) 1.08 * (1.01, 1.14)
Overall 1.21 *** (1.15, 1.27) 1.33 *** (1.25, 1.42) 1.06 * (1.00, 1.12)
O3 Lag 0-5 Lag 0-3 Lag 0-5
0-4 - 1.35 *** (1.20, 1.53) -
5-64 1.23 *** (1.11, 1.35) 1.33 *** (1.19, 1.49) 1.13 (0.98, 1.29)
65+ 1.31 *** (1.21, 1.41) 1.48 *** (1.33, 1.64) 1.13 ** (1.03, 1.24)
Overall 1.29 *** (1.21, 1.37) 1.39 *** (1.29, 1.50) 1.13 ** (1.04, 1.23)

* 0.05>p>0.01
** 0.01>p>0.001
*** p<0.001

   
5.9.2

Relative Risk Estimates of Hospital Deaths

Relative risks (RR) of hospital mortalities due to respiratory diseases, cardiovascular diseases and the two combined were estimated using the same model (Table 16). As before, the "best lag" (i.e., the parameter which had the highest chi-square value in the model) among the parameters of an individual pollutant was chosen to estimate the relative risk. RR of deaths due to respiratory diseases and to respiratory and cardiovascular diseases combined were significantly higher than unity for NO2 and O3, but not for RSP and SO2. The magnitude of the RR for respiratory mortalities was higher than for respiratory admissions for O3 (RR= 1.62), but lower for NO2 (RR= 1.31). With the exception of O3 (RR= 1.27), RRs of death due to cardiovascular diseases were not significantly higher than unity.

 
Table 16: Relative risks and 95% confidence intervals for 100 ug/m3 increase in the levels of air pollutants for deaths due to respiratory  diseases, cardiovascular diseases and combined (1994 - 95).
 
Relative risk (95% confidence intervals)
Pollutant Total hospital deaths
(respiratory + circulatory)
Respiratory Mortalities Cardiovascular Mortalities
NO2 Lag0-3#

1.31** (1.10-1.55)

Lag11.31**
(1.08-1.58)

Lag0-3#1.22
(0.97-1.54)

SO2 Lag0-1#

1.24 (0.99-1.55)

Lag0-1#1.35
(0.97-1.88)

Lag0-2#1.21
(0.85-1.71)

RSP Lag0-3#

1.13 (0.98-1.30)

Lag0-2#1.21
(0.99-1.48)

Lag31.06
(0.93-1.21)
O3(8h) Lag0-5#

1.44*** (1.20-1.74)

Lag0-5#1.62***
(1.23-2.14)

Lag51.27**
(1.09-1.48)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

# Lag0-1, 0-2, 0-3 and 0-5 indicates cumulative lags for the respective days.

   
5.10

Risk Estimates using the Multiple Pollutant Model

Tables 17 to 19 summarizes significant main effects (pollutants) and interactions for total (respiratory and circulatory diseases) admissions, admissions for respiratory diseases, and admissions for circulatory diseases. In the multi-pollutant model, SO2 was not significant and therefore not included in the stepwise selection process.

For total (respiratory and circulatory) admissions, significant pollutants were RSP (lag 0-3), NO2 (lag 0-1) and O3 (lag 0-5). As there was significant interaction between NO2 and O3, their respective RRs were expressed at three arbitrary levels of each other, namely the 25th percentile, the median (50th percentile) and the 75th percentile of the levels of pollutants (Table 17). A synergistic effect between the two pollutants was observed (i.e., the RR of each pollutant was higher at a higher level of the other pollutant than at a lower level). Compared with the single pollutant model, the magnitude of the RRs for RSP, NO2 and O3 was generally smaller.

 
Table 17: Relative risks for RSP, NO2 and O3 for total (respiratory and circulatory) admissions

A. RSP

RR for RSP 95% CI
1.08*** (1.01, 1.15)


B. NO2 (at different levels of O3)

O3 level RR for NO2 95% CI
25% (16.0) 1.03 (0.95, 1.11)
50% (24.4) 1.08* (1.01, 1.15)
75% (38.3) 1.16*** (1.08, 1.25)


C. O3 (at different levels of NO2)

NO2 level RR for O3 95% CI
25% (41.9) 1.06 (0.97, 1.16)
50% (51.3) 1.12** (1.04, 1.21)
75% (66.6) 1.22*** (1.14, 1.31)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

For respiratory admissions, the following were significant pollutants and interaction terms:- RSP (lag 0-3), NO2 (lag 0-3), O3 (lag 0-3), RSP with O3, and NO2 with O3 (Table 18). As O3 interacted with both RSP and NO2, the RRs of ozone were expressed at three different levels of NO2 and of RSP. At every level of RSP, the RR of O3 (albeit insignificant in some cases) increased with higher levels of NO2. The marginally significant RR of 0.84 for NO2 at 25% of O3 was probably a spurious finding. The interaction of RSP with O3 was antagonistic, i.e., the RR for RSP decreased with increasing levels of O3, while the RR for O3 decreased with increasing levels of RSP at every level of NO2. When interpreting the RR of O3, it should be noted that, as RSP and NO2 were correlated, there were few days in the dataset with high levels (75th percentile and above) of one pollutant and low levels (below 25th percentile) of the other.

 
Table 18: Relative risks for RSP, NO2 and O3 for respiratory admissions

A. RSP (at different levels of O3)

O3 level RR for RSP 95% CI
25% (15.0) 1.43*** (1.23, 1.66)
50% (24.3) 1.33*** (1.18, 1.49)
75% (39.8) 1.17** (1.06, 1.29)


B. NO2 (at different levels of O3)

O3 level RR for NO2 95% CI
25% (15.0) 0.84* (0.70, 0.99)
50% (24.3) 0.94 (0.81, 1.08)
75% (39.8) 1.14 (0.99, 1.30)


C. O3 (at different levels of RSP and NO2)

NO2
25% (42.5) 50% (52.0) 75% (66.5)
  25% (33.5) 1.20**
(1.07, 1.34)
1.35***
(1.20, 1.51)
1.61***
(1.37, 1.89)
RSP 50% (44.9) 1.09
(0.97, 1.22)
1.23***
(1.12, 1.35)
1.47***
(1.30, 1.66)
  75% (62.8) 0.94
(0.80, 1.11)
1.06
(0.94, 1.19)
1.27***
(1.15, 1.39)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

For circulatory diseases admissions, the following pollutants and their interaction were significant: NO2 (lag 0-1), O3 (lag 0-5), NO2 with O3 (Table 19). As before, NO2 and O3 showed synergistic interactions, but reached significance only at higher pollutant levels.

 
Table 19: Relative risks for NO2 and O3 for circulatory admissions

A. NO2 (at different levels of O3)

O3 level RR for NO2 95% CI
25% (16.0) 1.04 (0.95, 1.13)
50% (24.4) 1.09* (1.01, 1.17)
75% (38.3) 1.17*** (1.08, 1.27)


B. O3 (at different levels of NO2)

NO2 level RR for O3 95% CI
25% (41.9) 0.97 (0.86, 1.08)
50% (51.3) 1.02 (0.93, 1.12)
75% (66.6) 1.11* (1.01, 1.21)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

The multiple pollutant model was also applied to different age groups (Tables 20 to 25). For the age group 0-4, respiratory admissions, RSP (lag 0-3) and O3 (lag 0-3) were the only significant terms in the model. No significant interaction was found. As the number of admissions for circulatory diseases for this age group was comparatively small (420 vs. 30,263 for respiratory admissions), the RRs were not computed.

 
Table 20: Relative risks for RSP and O3 for respiratory admissions (aged 0-4)
 
  RR  95% CI
RSP 1.33*** (1.18, 1.49)
O3 1.15* (1.01, 1.32)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

When analyzing total hospital admissions (respiratory and circulatory diseases combined) among the 5 to 64 years age group (Table 21), the following parameters were selected, NO2 (lag 0-1), O3 (lag 0-5) and their interaction. Similar to the findings in all ages combined, a synergistic effect between NO2 and O3 was evident.

 
Table 21: Relative risks for NO2 and O3 for total admissions (aged 5-64)

A. NO2 (at different levels of O3)

O3 level RR for NO2 95% CI
25% (16.0) 1.05 (0.95, 1.17)
50% (24.4) 1.11* (1.02, 1.21)
75% (38.3) 1.20*** (1.09, 1.32)


B. O3 (at different levels of NO2)

NO2 level RR for O3 95% CI
25% (41.9) 1.03 (0.90, 1.18)
50% (51.3) 1.09 (0.97, 1.22)
75% (66.6) 1.19** (1.07, 1.33)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

For respiratory admissions (Table 22), the following were included in the model, RSP (lag 0-3), SO2 (lag 0), O3 (lag 0-3), and the interaction between SO2 and O3. It should be noted that SO2 (lag 0) was included despite being insignificant by itself, because the interaction term was significant. The effect of O3 was enhanced at higher levels of SO2. For circulatory diseases admissions, no pollutants were selected in the model.

 
Table 22: Relative risk for RSP, SO2 and O3 for respiratory admissions (aged 5-64)

A. RSP

RR for RSP 95% CI
1.19** (1.07, 1.34)
 
Table 22:
(cont'd)
Relative risk for RSP, SO2 and O3 for respiratory admissions (aged 5-64)

B. SO2 (at different levels of O3)

O3 level RR for SO2 95% CI
25% (15.0) 0.91 (0.76, 1.07)
50% (24.3) 1.00 (0.87, 1.14)
75% (39.8) 1.17 (0.97, 1.40)


C. O3 (at different levels of SO2)

SO2 level RR for O3 95% CI
25% (12.4) 1.11 (0.96, 1.29)
50% (17.1) 1.17* (1.02, 1.33)
75% (25.0) 1.27*** (1.10, 1.45)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

Among those aged 65 years and above, for total (respiratory and circulatory diseases) admissions, the selected parameters were SO2 (lag 0), NO2 (lag 0-1), O3 (lag 0-5) and the interaction term NO2 with O3 (Table 23). The interaction was synergistic, as in the 5-64 age group. SO2, which was not selected in the multiple pollutant model in the younger age groups or in all ages combined, was a significant pollutant in this age group.

 
Table 23: Relative risks for SO2, NO2 and O3 for total admissions (aged 65 and above)

A. SO2

RR for SO2 95% CI
1.19*** (1.08, 1.30)


B. NO2 (at different levels of O3)

O3 level RR for NO2 95% CI
25% (16.0) 0.98 (0.90, 1.08)
50% (24.4) 1.04 (0.96, 1.13)
75% (38.3) 1.15** (1.05, 1.25)


C. O3 (at different levels of NO2)

NO2 level RR for O3 95% CI
25% (41.9) 1.11 (0.99, 1.23)
50% (51.3) 1.18*** (1.08, 1.29)
75% (66.6) 1.31*** (1.20, 1.43)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

For respiratory diseases admissions, significant terms were: pollutants - SO2 (lag 0), RSP (lag 0-3), NO2 (lag 0-3) and O3 (lag 0-3); interactions - RSP with O3 and NO2 with O3 (Table 24). As with total admissions, SO2 was a significant pollutant. Similar to findings for all ages combined, RSP interacted antagonistically with O3, and synergistically with NO2. However, RR for the latter was found to be (marginally) below unity at low levels of O3 (25%).

 
Table 24: Relative risks for SO2, RSP, NO2 and O3 for respiratory admissions (aged 65 and above)

A. SO2

RR for SO2 95% CI
1.24*** (1.10, 1.41)


B. RSP (at different levels of O3)

O3 level RR for RSP 95% CI
25% (15.0) 1.29* (1.06, 1.59)
50% (24.3) 1.20* (1.02, 1.41)
75% (39.8) 1.05 (0.92, 1.20)


C. NO2 (at different levels of O3)

O3 level RR for NO2 95% CI
25% (15.0) 0.78* (0.61, 0.99)
50% (24.3) 0.91 (0.74, 1.11)
75% (39.8) 1.16 (0.96, 1.41)


D. O3 (at different levels of RSP and NO2)

NO2
25% (42.5) 50% (52.0) 75% (66.5)
  25% (33.5) 1.29**
(1.01, 1.51)
1.50***
(1.28, 1.76)
1.89***
(1.51, 2.38)
RSP 50% (44.9) 1.17
(0.99, 1.38)
1.36***
(1.19, 1.56)
1.72***
(1.45, 2.04)
  75% (62.8) 1.00
(0.80, 1.26)
1.17
(0.99, 1.38)
1.48***
(1.29, 1.68)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

For circulatory diseases admissions, NO2, O3 and their interaction term were entered in the model (Table 25). (As the interaction term was significant, O3 was included in the model, despite the fact that it was not significant by itself). As before, the interaction between NO2 and O3 was synergistic.

 
Table 25: Relative risks for NO2 and O3 for circulatory admissions (aged 65 and above)

A. NO2 (at different levels of O3)

O3 level RR for NO2 95% CI
25% (16.0) 1.07 (0.98, 1.18)
50% (24.4) 1.13** (1.04, 1.22)
75% (38.3) 1.23*** (1.13, 1.34)


B. O3 (at different levels of NO2)

NO2 level RR for O3 95% CI
25% (41.9) 0.93 (0.82, 1.06)
50% (51.3) 0.99 (0.89, 1.10)
75% (66.6) 1.09 (0.98, 1.20)

* 0.05 p 0.01
** 0.01 p 0.001
*** p < 0.001

   
5.11

Validation of the Model with 1996 Dataset

Time series plots of the observed total (respiratory and cardiovascular) hospital admissions in 1996 and the predicted numbers based on the 1994 - 95 model were shown in Figures 18 to 21. On visual inspection of the graphs, the predicted values were found to generally coincide with the peaks and troughs of the observed values. A systematic downward bias in total (respiratory and cardiovascular) admissions was observed for all four air pollutants in the last four months of the first half-year (i.e., the "observed" values being higher than the "expected" values). However, the model was found to fit fairly well during the first two months of 1996.


View Graphs:

  1. Figure 18: Observed vs. predicted hospital admissions (respiratory + cardiovascular) for 1996 (SO2 lag0)
  2. Figure 19: Observed vs. predicted hospital admissions (respiratory + cardiovascular) for 1996 (O3 lag0-5)
  3. Figure 20: Observed vs. predicted hospital admissions (respiratory + cardiovascular) for 1996 (NO2 lag0-1)
  4. Figure 21: Observed vs. predicted hospital admissions (respiratory + cardiovascular) for 1996 (RSP  lag0-3)

 

 

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