I'm examining several pollutant variables -- like CO2, lead, NOX etc -- and wondering how I can weight them appropriately to have some kind of comparability. Not all pollutants are equally bad for the environment and some have only local rather than global impacts.

Could one transfer a ton of pollutant X and pollutant Y into a common comparison of say implied mortality costs, CO2 footprint equivalents?

is there a literature on this, or some widely used methods?

  • 1
    $\begingroup$ I am aware that it is commonly done for the purposes of tracking GHG emissions, by converting emissions of the relevant gasses to CO₂ equivalent (CO₂e). For the United States, the computation is specified in this regulation, equation A-1 and table A-1. $\endgroup$
    – njuffa
    Oct 13 at 19:37
  • $\begingroup$ many thanks Njuffa $\endgroup$
    – cel
    Oct 14 at 8:59

2 Answers 2


It’s not exactly what you’re asking for but quality index is the closest that comes to mind. https://www.airnow.gov/aqi/aqi-basics/.

There are so many variables, it is difficult to develop one accurate unit of measure. For instance, high pollen count or high concentrations of an allergen could be deadly for some people and harmless to others. Different combinations and/or attributes of pollutants might be deadly where as a small change to them might make them harmless.

if the particulates in your particle size count were silica dust causing lung abrasions it could amplify the effects of something like formaldehyde. But the harm caused by the same concentration of formaldehyde and the same particle size count would be totally different if those particles were just dust from soil.


Climate pollutants and air-quality pollutants would not be part of the same index, except perhaps total "economic costs". Climate pollutants can be lumped into a CO2-equivalent value, which relies on the residence time and warming potential.

For criteria air pollutants that are regularly monitored (NO2, CO, O3, PM2.5, SO2), EPA uses a method which reports the air quality index for the worst pollutant. For toxic air pollutants, air quality agencies have typically done the same thing, just pick the worst one.

Health based standards are usually expressed in terms of the cancer or mortality probability for a single pollutant. However, cumulative and combined exposure to multiple air pollutants can have more negative health effects than just "the worst one", or even worse than the simple additive effects across all pollutants. It is very difficult to come up with a cumulative and combined health metric because most health studies don't have that type of robust information. But, we do know that the health effects are compounded when their are multiple stressors. In order to calculate the cumulative health impacts of multiple air pollutants, some people use BenMAP, which I believe incorporates the Cox proportional hazards model to make the calculation.

The Cox proportional-hazards model (Cox, 1972) is essentially a [statistical] regression model commonly used in medical research for investigating the association between the survival time of patients and one or more predictor variables.

You might find other useful models at the EPA's Air: Fate, Exposure and Risk Analysis site.

Environmental groups are pushing air quality agencies to now consider cumulative exposure when permitting facilities, especially under the environmental justice context. See Oregon's Cumulative Health Risk pilot study for toxics, as an example.


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