(FORTUNE Small Business) – Next time an SUV roars past, don't forget to thank its driver for helping the environment--assuming the vehicle sports a TerraPass bumper sticker. That accessory means that the owner has agreed to pay as much as $80 a year to Benven, a student-owned business in Philadelphia. The company invests the money in various clean-air projects. About 500 drivers have signed up, and the for-profit venture raised $50,000 from angel investors. "Environmental activists who hate business hate our product," admits Wharton professor Karl Ulrich, who inspired the idea. --I.M.