A ban on the use of wood stoves and fireplaces will go into effect in Denver on days when the air looks bad, not just when the carbon monoxide gauges say it is.

The new regulations go into effect Thursday. The restrictions on burning wood are not new; Denver has had them for years. It's the criteria for determining when to impose the restrictions that are being changed.Previously, the restrictions went into effect when carbon monoxide exceeded federal levels. But officials got tired of getting complaints they were not doing enough to eliminate the pollution that hangs over the city during winter.

"We had a credibility problem," John Leary, deputy director of the air pollution control division of the state Health Department, said Tuesday. "We'd say there was no high pollution day, and people would look out and it looked terrible."

It conducted a survey, asking 200 people chosen at random to look at slides and decide which showed dirty air.

"We urged people to consider that they live in an urban area and not to expect the pristine air, not what they would find in a national park. There was remarkable agreement, and at that point a standard was set," Leary said.

They agreed that the air is polluted even when 93.6 percent of a beam of light gets through a kilometer of air, far tougher than federal standards.

A pollution alert triggers a wood-burning ban that should idle most wood-burning stoves and fireplaces. Smoke from wood fires plays a significant role in Denver visibility, which gets dirty brown air in the winter.

Violators of a pollution alert can be fined $25 or more for repeat offenses.