A new report from the US National Research Council (NRC) said that short term exposure to ozone, a key component of smog, is likely to contribute to premature deaths.
Ozone damages human health, causing respiratory and other problems, said the committee that wrote the report. They also said there was mounting evidence of a link between short term exposure to ozone (that is less than 24 hours) and deaths. However, the EPA asked them to produce the report because the evidence has been interpreted differently by different bodies.
The EPA was particularly keen for the NRC to analyze the link between ozone and mortality and to express lives saved in money terms, in order to compile more meaningful health-benefits assessments. The EPA is charged with setting “acceptable” ozone levels in US cities.
The NRC Committee on Estimating Mortality Risk Reduction Benefits from Decreasing Tropospheric Ozone Exposure, reviewed recent research and found that deaths linked to ozone exposure were more common among people who were already ill or susceptible for other reasons and that premature deaths were not limited just to those who were already within a few days of dying.
The committee were particularly interested in finding out if there was a threshold level of ozone concentration in the air below which there was no link to increased risk of death. They reviewed research done on large populations that looked at different ozone concentration effects on deaths.
They did not find a universal threshold, but said that if it exists, it lies below the current public health standard. The NRC committee said more research was needed on personal thresholds, since sensitivity to changes in ozone levels varies by individual and not everyone’s risk of death changes with the same change in ozone concentration. How frail a person is may also play a role, they said.
The committee found that the available research on short term exposure accounts for only some ozone-linked mortality. The risk of death may be greater, if longer term exposure, such as weeks and years, is taken into account, and more research is needed in this area, and the link with life expectancy should also be explored they said. It is possible for example, that some deaths linked to short term exposure don’t occur until several days later, and then there is the case of multiple short term exposures, which is currently not well served by research.
The NRC said the EPA should do more to understand the regional and seasonal differences in ozone levels, and monitor it during the winter when it is low.
They also recommended more studies be done on how other pollutants such as microscopic particles in the air affect the ozone-mortality relationship.
The EPA uses a ratio called the value of a statistical life (VSL) for cost-benefit analyses of improved health outcomes. The VSL is a monetary value derived from studies of adults who indicated what they would be willing to pay to change their risk of death in a given period by a small amount: based on what benefits or conveniences they would be willing to forego.
The EPA uses VSL as a way to express lives saved in monetary terms so it can be balanced in the same “currency” as other items in the cost-benefit analysis. VSL can be used to roughly work out if something is worth doing, for example because it extends life expectancy. However, it is difficult to establish the rules for how to apply VSL across age groups and whether to distinguish between people who are very sick and those who are healthy.
At the moment the EPA does not distinguish between these groups and applies the VSL to all lives saved regardless of age or health status.
The committee concluded that the EPA should not change this rule, because the current evidence is not enough to help produce a better way of doing it, for example according to differences in remaining life expectancy and health status. But the committee did not rule out that such adjustments might be appropriate in the future, for example by adjusting VSL to express value of remaining life, and thus apply it differently to a sick 80 year old person compared to a healthy 2 year old. More research was needed to address the questions about the current EPA approach to the use of VSL, said the NRC committee.
“Estimating Mortality Risk Reduction and Economic Benefits from Controlling Ozone Air Pollution.”
Committee on Estimating Mortality Risk Reduction Benefits from Decreasing Tropospheric Ozone Exposure, National Research Council.
Washington: The National Academies Press, 2008.
ISBN-10: 0-309-11994-4
ISBN-13: 978-0-309-11994-8
Click here for the full report (free to read online).
Sources: The National Academies press release.
Written by: Catharine Paddock, PhD