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Background Concentrations


For six of the 1999 air toxics of concern in New Jersey, NATA predicts air concentrations that are higher than their respective health benchmarks even without local emissions from within New Jersey. These six pollutants are:

  • Benzene
  • Ethylene dibromide
  • Carbon tetrachloride 
  • Ethylene dichloride
  • Chloroform
  • Formaldehyde
  • For these pollutants, USEPA assumed a constant background concentration for the whole country. For the purposes of the NATA study, "background" represents air toxic concentrations resulting from emissions from past years (persistence), and long-range transport from distant sources.

    Comparing the background concentrations of these pollutants to their health benchmarks results in risk ratios greater than one. This is shown graphically in Figure 1.

    Figure 1: Risk Ratios of Background Concentration Over Health Benchmark
    Figure 1: Risk Ratios of Background Concentration Over Health Benchmark

    To gain a better understanding of the significance of these background concentrations, we have compared the percent contribution to total air concentration from background to the percent contribution from New Jersey major point, area, on-road mobile and non-road mobile sources. This distribution is shown in Figure 2.

    Figure 2: Source Category Contributions to Overall Concentrations

    From Figure 2, you can see that background is responsible for 100% of the predicted concentrations of carbon tetrachloride and ethylene dibromide, and over 90% of the predicted concentration of ethylene dichloride. For the other three pollutants, there are also significant contributions from other source categories. The biggest contribution to the total benzene concentration comes from the on-road mobile source category, with smaller contributions from all of the other source categories. Chloroform comes mainly from background, but about 10% of the predicted air concentration comes from major point sources and 10% from area sources. For formaldehyde, the biggest contribution is from non-road sources, followed closely by on-road sources. Area sources and background contribute smaller amounts.

    Figure 3 again shows the risk ratios for these six pollutants, but also shows the amounts attributed to different sources.

    Figure 3: Source Category Contributions to Risk Ratios
    Figure 3: Source Category Contributions to Risk Ratios

    Comparing this graph to Figure 1, we can see that the total risk ratios for carbon tetrachloride, ethylene dibromide and ethylene dichloride do not change much when other sources are added to Background. However, the benzene and formaldehyde risk ratios do increase substantially. The emissions from these two pollutants will require multiple control stategies if the risk ratios in the state are to be reduces. For the other four Background pollutants, there is little that New Jersey can do locally to reduce the general exposures experienced in this state.

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