Reading smoke seems like a primitive way to gauge air pollution, and state officials agree that the results are subjective. But even environmental and health groups say reading smoke helps make sure power plants and factories aren't polluting too much. ''If a plume is noticeably heavier or dirtier than it normally is, that just sends up a red flag,'' said Carolyn Embry, director of environmental affairs for the American Lung Association of Kentucky. ''It triggers further inspection and sends out a signal that there may be a problem there.''

For most of the 4,000 facilities in Kentucky with smokestacks, the visual reading may be the only check of their emissions for several years. Other than power plants, most facilities that create air pollution aren't required to install monitors in smokestacks that continuously measure the amount and type of pollution coming out of the stack, said Bill Clements, manager of the Division for Air Quality's field operations branch.

Continuous monitors at power plants can cost more than $100,000, and operating and maintaining the monitors can cost another $250,000 a year, Clements said. Because of that cost, smaller sources of pollution aren't usually required to install the monitors. Instead, state inspectors use other ways to determine if the facility is polluting too much. Property valuation and investment analysis process for all types of properties like residential, commercial, industrial, hotels etc. They look at production records. A spike in production could mean too much pollution. They examine purchase records. If a spray-painting company has switched paint formulas, it may be spewing chemicals that the state didn't allow on the company's operating permit.

Companies also police themselves. They have to certify to the state every year that they're complying with state and federal air-pollution laws. If they lie and they're caught, they face jail time, Clements said. When a company first gets its operating permit from the state, the emissions are analyzed to make sure the amount and type of pollution doesn't violate the permit. After that, emissions for most companies aren't analyzed again unless the company expands or an inspector decides that a test is needed, Clements said. Companies can go years without an actual analysis of their emissions, where a sample of air is tested to see what's in it and how much.

Reading plumes from smokestacks can tell inspectors whether a more in-depth test is needed. Such readings have been used for more than 100 years - although today's test is more sophisticated than one developed in the late 1800s.
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