Both the British Government and the new Republican leadership in the US Congress are trying to reduce controls on the grounds that they are a burden on industry.
But the latest research, in a report by the Institute for Economic and Environmental Studies at California State University - the most comprehensive ever undertaken - shows that even the industries which suffered the strictest controls to combat the region's notorious air pollution still outpaced their counterparts in cities with far weaker rules, while other businesses were drawn to the region by the cleaner air that resulted.
"The widespread concept is that environmental regulations are economy killers," said Jane Hall, professor of economics at the university and leader of the study, last week. "But we can find no evidence that this is the case. "This is so even though we have looked at the part of the world where the regulations have been most aggressive and where one might expect them to have had the greatest adverse effect."
In fact, she said, the regulations seemed to have had a beneficial effect. Over the Seventies and Eighties, when they took effect most strongly, manufacturing jobs grew by nearly 17 per cent in Southern California, while declining by 4 per cent in the rest of the country.
Over the past 40 years, the world's harshest measures have reduced air pollution by half and cut smog alerts by 86 per cent. The regulations continue to get tougher. By the end of the decade, makers of new cars will have to have cut emissions by 90 per cent, and the most polluting industries will have to achieve 70-80 per cent reductions by 2010.
Professor Hall's team looked in detail at the most severely regulated industries, which were widely predicted to decline as a result of the controls, and found that they were not seriously affected. In fact, Southern California's share of US jobs in petrol refining, chemicals, paper, metal, and stone, clay and glass manufacture - industries which shouldered most of the heaviest costs of the clean-up - has grown during the Eighties and Nineties.
The report says the growth of the area's manufacturing industries actually accelerated in the Eighties, the decade when the controls took their fullest effect.
A few industries, such as furniture-making and aircraft painting, may have lost jobs to other states, partly because of rules that restricted the use of paints and varnishes which give off fumes that contribute to smog.
But established companies and new industries alike benefited by making anti-pollution equipment. They found preventing pollution made processes cheaper, freeing money for investment and employment.