How do you combine different forms of pollution into a single number?

AutoEcoRating merges the disparate impacts of different pollutants by applying principles of lifecycle analysis and environmental economics. Two main steps are involved. The first is adding up the major non-global-warming impacts using what are known as environmental damage cost estimates. The second step combines those results with greenhouse gas (GHG) emissions in order to compute the single environmental impact metric that is subsequently converted into the eRating as explained in the “What do the eRating numbers mean?” FAQ.

For the first step, we use environmental damage cost estimates from a report by the U.S. Office of Management and Budget (OMB). These estimates, however, are only available for impacts that have already occurred (for which the extent of harm has been measured and monetized). Such damage costs are not available for GHG emissions because the greatest climate risks extend into the future. Therefore, the second step involves making an assumption about how to combine GHG emissions with other forms of pollution. AutoEcoRating does this by treating these two major types of environmental damage as being equally important. Otherwise put, we assume a “50-50” weighting for GHG emissions and other impacts. This is clearly a judgment call and we acknowledge that others may have different views on the relative importance of global warming compared to other impacts on public health and the environment. Of course, many other judgments are involved in applying the emissions factors and other parameters used in the algorithm, which draws upon the editor’s longstanding expertise on automotive environmental issues (see AutoEcoRating Background).

After applying these weighting factors, the AutoEcoRating algorithm calculates the total environmental impact in the units of tons of emissions averaged over a vehicle lifetime (assumed to be 180,000 miles). Technically, the units of this number are given as if they were metric tons of carbon dioxide (CO2). However, the resulting number is not the same as “tons of CO2-equivalent” as commonly discussed in the context of climate impact analysis; that is because it incorporates many non-global warming impacts based on the “50-50” weighting  just described.

Even though the units are average tons per year, AutoEcoRating’s impact metric is a purely relative indicator of environmental harm. For example, using the OMB-based damage costs and the “50-50” assumption for global warming vs. other impacts, the algorithm treats smog-forming nitrogen oxide (NOx) emissions as roughly 500 times more important than CO2 on a mass basis (i.e., a ton of NOx is treated as equally harmful as about 500 tons of CO2).

The result, for an average model year 2010 light vehicle (average of cars and light trucks such as minivans, SUVs and pickups), is a total impact of 11.5 tons per year. We call this the “Environmental Impact Rating” (EIR) and again, it represents a weighted average of many different forms of pollution and so should not be confused with a “carbon footprint” that looks only at CO2 emissions. By way of comparison, the “carbon footprint” computed directly from EPA data would be 4.7 tons per year, using the agency’s model year 2010 average CO2 emissions rate of 395 grams per mile and 12,000 miles of driving. That number is lower because it does not include as many impacts as we account for in AutoEcoRating.

Finally, we map the lower-is-better EIR on to a higher-is-better eRating by simply scaling its inverse. For model year 2010, the average EIR of 11.5 tons then translates to an average eRating of 100. That value is the point of reference when an AutoEcoRating article describes how much greener a given car or truck is compared to an average vehicle.


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