ANTHROPOGENIC semi-volatile organic compounds such as polycyclic aromatic hydrocarbons (PAHs) are highly lipophilic (which makes them likely to accumulate in animal tissue), and some are carcinogenic or mutagenic(1). Although such compounds are known to accumulate in vegetation(2-5), little is known about the quantitative role played by vegetation in removing them from the atmosphere. We have developed a mass-balance model for PAHs for the northeast of the United States, based on measurements of PAHs in soil and vegetation from Bloomington, Indiana, and published values for PAH concentrations and fluxes in air, water, sediments and soils. Our model shows that 44 +/- 18% of the PAHs emitted into the atmosphere from sources in this region are removed by vegetation. Although the equilibrium between the atmosphere and vegetation depends on ambient temperature(6), we believe that most of the PAHs absorbed by vegetation at the end of the growing season are incorporated into the soil(7,8) and permanently removed from the atmosphere.