Intake fraction, a metric that summarizes the emission-to-inhalation relationship, facilitates comparisons among sources in terms of their exposure potential. For a given emission source and pollutant, intake fraction is the cumulative mass inhaled by the exposed population divided by the cumulative emissions. One way to estimate the environmental health impact of a pollution source or source class is as the product of three terms: emission rate (mass per time), intake fraction (mass inhaled per mass emitted), and toxicity (health impact per mass inhaled). In the ideal situation, one would know all three terms for all major emission sources. However, important insight can be gained even without complete information. For example, if two sources are identical except that the intake fraction is twice as high for source A as for source B, then the health beneﬁt per mass emission reduction is expected to be twice as large for A as for B.