The Teamsters union and environmental activists have formed an unlikely and outspoken alliance aiming to clear the air in American ports, and perhaps bolster the Teamsters' ranks in the process.

The labor-green alliance is getting under the trucking industry's skin by asserting that short-haul trucking companies working in ports - and not the truck drivers, who are often considered independent contractors - should spend the billions needed to buy new, low-emission rigs that can cost $100,000 to $175,000 each.

The Teamsters union says seaport air is so dirty largely because port truck drivers earn too little to buy trucks that would belch out fewer diesel particulates, tiny particles that contribute to cancer and asthma. Working with environmentalists, the union helped persuade the Port of Los Angeles to adopt a far-reaching plan that bars old trucks from hauling cargo from the port and puts the burden of buying new vehicles on the trucking companies, not the drivers.

The battle has intensified as federal officials press ports to adhere to clean-air regulations. Seaports from Newark to Miami to Seattle are confronting the same issue: who should pay for the cleaner trucks?

Once again, many drivers are rushing to haul as many loads as they can each week, although they often wait for two unpaid hours to pick up loads. Many say they now earn $8 to $10 an hour. Mr. Holmes and many environmentalists ask who will buy the next generation of trucks in five years if it is left to the drivers.

"You can't get clean air on the backs of the drivers," said Amy Goldsmith, executive director of the New Jersey Environmental Federation. "They can't possibly earn enough the way the system is set up, with the drivers required to buy gas, insurance and equipment."