ANTECEDENTES DE LA UNIDAD DE ANÁLISIS O POBLACIÓN 29

Although it is generally agreed that the associations of both short-term and

long-term PM exposure with adverse health endpoints have been consistently observed,

and thus neither of them should be ruled out in risk assessment and management, there is

no consensus about their relationship and relative magnitude. Kunzli et al (2001) presents

the following conceptual framework to describe the relationship between the deaths

attributable to long-term exposures and those attributable to short-term exposures to fine

particles:

Note: (1) Source: Kunzli et al, 2001; (2) Circle sizes do not reflect relative effects.

Figure 2.2 Graphic Illustration of Deaths Due to Ambient Air Pollution in a Population

Based on this graph, the authors argued that deaths attributable to air pollution can

be categorized into four cases, corresponding to the letters A, B, C and D in the graph: A)

air pollution increases both the risk of underlying diseases leading to frailty and the short

term risk of death among the frail; B) air pollution increases the risk of chronic diseases

leading to frailty but is unrelated to timing of death; C) air pollution is unrelated to risk of

chronic diseases but short term exposure increases mortality among persons who are frail;

air pollution. And the authors also compared the differences of these four categories of

cases in the following table:

Table 2.7 A Comparison of the Four Categories of Death Cases Attributable to Air Pollution

Impact of Air Pollution Category of

Cases Underlying Frailty Due to Air Pollution

Occurrence of Death (Event) Triggered by Air Pollution

A Yes Yes

B Yes No

C No Yes

D No No

Source: Kunzli et al, 2001.

Overall, it is difficult to derive an estimate of the total annualized mortality effect

due to reductions in ambient PM levels that may reflect reductions in both short-term

peak and long-term average exposures to PM (Industrial Economics Inc, 2006).

Time-series studies analyze the impacts of daily variations in PM concentrations and can

characterize the cumulative impact of exposure over a few days, but not over a longer

period of time. And uncertainty also remains in the issues such as the time-lag of effects

after exposure and the spread period of effects for short-term effects. Different studies

have reported different time lag (usually spans from 0 to 5 days) and studies also showed

that the effect was spread over several days and did not reach zero until a few days after

the exposure (e.g. Schwartz, 2000). A very recent study argued that air pollution exposure

with comparable total daily dose may have very different effects when occurring at high

levels over a few hours as opposed to low levels over a longer time, implying that the

effects of airborne PM on daily mortality may be underestimated, because the daily

average describes chronic exposures but does not capture information about acute

2005).

On the other hand, cohort studies focus primarily on analyzing the impact of

long-term exposures to PM but may also capture some of the impact of short-term

variations in exposure during the cohort follow-up period (Industrial Economics Inc,

2006). In addition, although cohort studies have indicated larger effects on incremental

mortality vis-à-vis short-term exposure, the actual effect is still not clear due to some

limitations in the study design. For instance, EPA’s criteria document argued that the

chronic exposure relative risk estimates are based on PM concentration during the 5 or 15

to 20 years study periods and do not necessary reflect the full impacts of longer past PM

exposure, which was likely to be much higher in the most highly polluted cities, resulting

in overestimates of the relative risk attributable to long-term exposure to PM air pollution

(EPA, 2004).

This study considers both the short-term and long-term effects of PM exposure on

mortality. However, due to the factor that only short-term (time-series) studies on the

association between mortality and PM levels are readily available in Thailand and the

transferability of epidemiological studies from developed countries to developing

countries is always an issue of concern, this study relies more on time-series studies

conducted in Thailand. In sensitivity analysis, long-term effects are included using cohort

studies performed in the U.S. and the magnitudes of both effects are compared. With

respect to the long-term effects, given that the U.S.EPA recently revoked the annual PM10

standard (effective December 17, 2006) due to a lack of evidence linking health problems

to long-term exposure to coarse particle pollution7, only the change in mortality

attributable to PM2.5is considered here. Given that PM2.5concentrations were not

measured in Thailand for the baseline year 2000, the mass concentrations of PM2.5are

7_{Source: U.S. EPA, National Ambient Air Quality Standards (NAAQS),http://www.epa.gov/air/criteria.html,} accessed November 9, 2006.

approximated by abstracting a reasonable ratio of PM10/PM2.5from the literature.