Dr CM Wong
(Data analysis and report writing)

Mr Stefan Ma
(Computation and statistical advice)

Materials and methods

Findings

Discussion

Conclusions

For hospital admissions

Air pollution was found to have an effect on circulatory and respiratory diseases combined and separately (relative risk, RR=1.03-1.10; p<0.084) for all ages; on circulatory admissions (RR=1.05-1.10; p<0.001) for the 65 or above age group; on respiratory admissions, the effects of which appeared to be j-shaped from the younger to the older age groups; on asthma (RR=1.10-1.16; p<0.018, except SO₂); on chronic obstructive pulmonary disease (RR=1.08-1.14; p<0.0001); and on ischaemic heart disease (RR=1.04-1.09; p<0.051, except SO₂ and RSP).

For hospital deaths

Both NO₂ and O₃ were positively associated with circulatory and respiratory diseases combined and separately (for NO₂: RR= 1.10-1.14, p<0.038; and for O₃: RR=1.07-1.22, p<0.010).

Validation and composite score

The above estimates were consistent with and in between those obtained from similar studies using the European (APHEA) approach overseas and in the CUHK. But in addition, a composite score was derived from the four pollutants and was found to provide consistent estimates for all the health outcomes under study in all ages (RR=1.04-1.11, p<0.098, except hospital deaths due to circulatory diseases).

Several epidemiological studies have now shown an association between particulate air pollution and exacerbations of illness in individuals with respiratory disease and also increases in the numbers of deaths from cardiovascular and respiratory disease, particularly in the elderly. New hypotheses have been advanced to postulate mechanisms underlying these observed effects (Seaton et al 1995).¹

Respirable particulates (RSP) with an aerodynamic diameter of <10 um(or particles measured as black smoke by the smoke stain method) comprise the principal pollutant associated with these findings. In addition RSP sulphate and sulphur dioxide concentrations, are reported to be associated with all causes mortality and respiratory mortality in recent studies from the USA (Pope et al 1995)² and the UK (Anderson et al 1996)³ respectively. A recent review in the United Kingdom concludes that the associations between daily concentrations of particles and acute health effects principally reflect a causal relationship (Committee on the Medical Effects of Air Pollutants 1995).⁴ After a lengthy scientific review, the USEPA determined that new standard should be added for particulates less than 2.5 ? of aerodynamic in size and the welfare-base standards were also revised by making them identical to the health-based standards.⁵

In Hong Kong we have found that SO2, RSP and SO4 concentrations are associated with excess risks for symptoms of cough, phlegm and wheeze and also bronchial hyper-responsiveness (by histamine challenge test) in primary school children (Hedley et al 1993; Peters et al 1996; Tam et al 1994; Wong et al 1998).^6,7,8,9 However in the London study the strongest association with daily mortality was for ozone. The effects of ozone and black smoke were independent of the effects of other pollutants.

Evidence on the risks associated with other pollutants is variable and less consistent. In the recent London study (Anderson et al 1996)³ the NO2 (1 hour maximum) was associated with all causes mortality and cardiovascular mortality; however a negative effect was seen for respiratory mortality. A significant positive effect on mortality was seen for SO2 and all cause mortality in the warm season period.

Several authors point to the complex between-season covariation of several pollutants, which is sometimes negative and at other times positive.

Study design

It was an ecological study utilizing routinely collected hospital admission data, air pollutant concentration data and weather data by the Hospital Authority, Environmental Protection Department and the Observatory respectively. Variations in the daily number of hospital admissions due to circulatory and respiratory diseases were studied, and their relationships with each of the pollutants were modelled to assess the effects of air pollution on health after adjustment for time trends, seasonality, weather conditions and some other factors including days of week, holidays and days after holidays.

This study follows a previous one performed by the Chinese University of Hong Kong (CUHK)¹¹ which followed the general approach of the protocol of the APHEA (a European approach using epidemiological time series data), developed within the frame of the EC Environment 1991-94 Programme. However data for some disease categories (ICD Rubrics) included in the APHEA protocol were not analysed in the CUHK study. The data sets and disease categories used for this new study are shown in Table 1 below. Those categories which were excluded or missing from the CUHK study are indicated in the table.

* first half year
** data for ICD9 390-392, 393-398, 446-448, 451-459, 470 and 510-519 were not included for analysis in the CUHK Final Report
*** data for ICD9 401-405 and 500-508 were missing in the CUHK data files and were not analysed in the CUHK Final Report
( ) CUHK data in brackets
- not included for analysis in the CUHK study

In order not to exclude categories which might show effects from air pollution (e.g. hospital admissions for cardiovascular diseases under ICD9 390 - 429 which were found to be related to respirable suspended particulates and carbon monoxide by Schwartz (1997)¹³ but not all included in the CUHK study) and to ensure comparability to the results of those studies which follow the APHEA protocol (Bacharova 1996; Schouten 1996),^14,15 we decided to follow strictly the categories recommend by the APHEA protocol for this study.

Data for 1995-1996 (generated and cleaned by the Department of Community Medicine, the University of Hong Kong in this study) were used in establishing the statistical models and in estimating the effects; and data for first half year of 1997 were used for validation of the established models. It was also decided that data for the year 1994, which were used by the CUHK group, should not be used in this study because the data quality is not consistent with that of the 1995-96 data set to be used in this study. In 1994 only 3 of the 12 hospitals under study had already adopted the MRAS database; but the number increased to 7 in 1995. Besides, the percentages of valid daily data for air pollutant concentrations from the monitoring stations were lower in 1994 than those in the other years.

Databases

Hospital admission data: Hospitals included for generation of hospital admissions were the publicly funded hospitals (accounting for 90% of hospital beds in Hong Kong) which either had an accident and emergency department, or was a referral base from the accident and emergency department of another nearby hospital (9) or had a 24-hour outpatient department (2). One other hospital which was the only hospital in the most polluted district in Hong Kong, was also included. All hospitals should have a computerized system for inputting and retrieval of patient data. The hospitals included in the study, together with the type of information system they were using and an indication as to whether they had an A & E department, were listed in Table 2 below:

* IPAS - Integrated Patient Administrative System
MRAS - Medical Records Abstracting System

Patients admitted to hospitals between 1.1.1995 and 30.6.1997 with data on: dates of admission and discharge, socio-demographic information (age, gender, marital status, ethnic group, district of residence, pseudo identifier of patient), admission source, discharge diagnosis in ICD9 codes and discharge status were retrieved from the databases for each of the hospitals under study. In order to validate the completeness of the retrieved data, the total number of inpatients for the period 1.4.1995 to 31.3.1996, for each hospital, were compared with those reported in the Hospital Authority Statistical Report 1995/96. When there were big differences between the two, the Hospital Authority Information Technology Department was ask for an explanation and we then revised the databases if necessary until they were reasonably close to each other. This process took almost a half year to complete. The data are shown in Table 3 below:

# total hospital patient discharges
@ The excess 5409 cases in the HKU data set were day cases which could not be excluded when the Information Technology Department generated the data set due to missing of the identifier.

The total data sets were then extracted for circulatory (ICD9 390-459) and respiratory (ICD9 460-519) diseases. The numbers of hospital admissions by disease groups in the three years were as shown in the previous Table 1 and subset of specific disease categories are shown in Table 4 below:

* first half year

Acute myocardial infarction was not analysed, as had been done by the CUHK, because diagnosis for the disease has been changing and subject to misclassification over the past years.

Pollutant concentration data: Pollutant concentration data in CD-ROM were made available by the Air Services Group of the Environmental Protection Department with hourly data from all monitoring stations in Hong Kong. The following stations in various urban, suburban and industrial areas were included in the study:

Two stations, one in Yuen Long (in suburban area) due to the extent of missing data and one in Mong Kok (in urban area) which provided concentrations measured on ground level, were excluded from the study. The pollutants included in this study are in Table 6 below:

* TEOM - Tapered Element Oscillating Microbalance, an instrument for the continuous measurement of particulates matter in air

In order to maintain consistency and similar quality standards in the data, missing data were defined and replaced in accordance with the APHEA recommendations. The guidelines were slightly modified to suit local situations and these are described in Table 7 below:

Note: RSP measured by TEOM.

# A minimum of 67% non-missing daily data was used as criteria for inclusion of a pollutant in the analysis instead of 75%. This was set in the computer programme at the beginning of the study when the HKU was trying to adopt a similar procedure as that used by the CUHK. However this might not be necessary for data after 1994 as the data were more complete.

The data in monthly averages were comparable to those in the EPD 1995 and 1996 statistical reports. The percentage of data valid after replacement are shown in the following Table 8:

* the data are available only for first half year of 1997
- not available
# Abbreviations referred to Table 5.

A daily mean concentration