Monday, January 3, 2011

Traffic, Pollution and Scales of Exposure

Traffic-Related Air Pollution: A Critical Review of the Literature on Emissions, Exposure, and Health Effects: 3.1 Introduction, page 3-3 (386 page pdf, Health Effects Institute,  Jan.12, 2010)

Also discussed here: Traffic-Related Air Pollution Literature Review (Pollution Free Cities, May 10, 2010)

Spatial and temporal scales are extremely important aspects to assessing exposure to traffic. While the greatest exposure is often at the smallest scale, the focus of much of the modelling (particularly by earth orbiting satellites) and monitoring or modelling of air pollutants (particularly by global numerical weather prediction models) is at the regional or global scale in tens or hundreds of metres, while the health impacts are at the human one metre scale. Bridging the difference remains the challenge for those seeking links between traffic, pollution and health as we are trying to do on this blog.

Key Quotes:

“Traffic-related emissions contribute to both primary and secondary ambient pollutant concentrations against a background of similar contaminants emitted from stationary (point and area) sources”

“the highest direct exposures to traffic-related emissions are likely to occur at the local scale, that is, in a vehicle traveling in traffic or on a roadside out to a few hundred meters. At greater distances (“urban scale”), traffic emissions are likely to be well mixed with emissions from other sources and are more difficult to apportions”

“The populations potentially at greatest health risk are people who either spend a considerable amount of time in traffic (such as taxi or truck drivers, commuters, and school children) or who live or work near busy roads”

“The identification of surrogates is a reasonable compromise in apportioning the contribution of traffic emissions to ambient air pollution and for use as a traffic-exposure measure in epidemiologic studies”

“Two broad categories of surrogates:

  1. measurement of traffic-related pollutants (carbon monoxide, nitrogen dioxide, benzene, particulate matter..)

  2. direct measures of traffic itself (traffic intensity, distance to the nearest road, traffic volume within buffers..)

“Central air-pollutant monitoring stations [EPA].. for monitoring criteria pollutants, have been placed to measure population exposures on the urban or regional scales and have produced short- and long-term data on the temporal variability of selected air pollutants”

“Although the measurements at these stations are impacted by traffic-related pollutants, the stations are not able to capture local-scale perturbations caused by such pollutants.….some countries in Europe using roadside monitors, which may capture small-area influences from traffic. Differences in monitoring practices complicate international comparisons of ambient conditions and exposures”

“In assessing the role of vehicle emissions on ambient pollutant concentrations and ultimately individual exposures, it is important to account for the temporal impact of the emissions, particularly on the local scale”

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  1. This is a very important topic. While the health impacts are felt at the 1 meter scale, it is important to also understand the limitations on the available or possible information.

    1. We have very limited number of monitors in the cities, which are most often used for regulatory purposes, not necessary at the (true) ground level (for various reasons). This puts a serious dent in the analysis of the personal exposure and linked impacts.

    2. Generally, if one starts apportioning the impacts of air pollution, the transport sector tends to be higher in the cities, mostly because of the low lying sources, but we shouldn't neglect the non-transport sectors such as garbage burning, fuel used for cooking and heating (especially in the cold climates), and of course the industries.

    3. There are new studies coming up, using the intake fractions, based on on-road measurements, population density, and vehicle densities, to estimate the exposure times - another way to estimate health impacts, besides using the monitoring data from the regular stations - which would most often represent the averages.

    All of these come with some level uncertainties and the positive side is something is better than nothing.

  2. Agree with you on all counts
    Solutions start with roadside emission measurements, using appropriate technologies such as the AirPointer and satellite AQ imagery calibrated to high resolution maps (see "Reports" on this blog for recent studies)
    DEFRA's Local AQ agreements with municipalities in the UK (with huindreds of monitors in place) appear to be an effective way of controlling the transportation side - and this has been demonstrated after more than 10 years of enforcement and complementary studies by public health authorities in that country showing relationships between health impacts and heavy traffic

    Are you aware of specific studies elsewhere?