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.

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.

* first half-year

* first half-year

* first half-year

first half-year

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.

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).

* Total = Respiratory diseases + Cardiovascular diseases

** Acute myocardial infarction

* 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

Age group

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).

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.