It's pretty much a given that anti-smoking laws lead to cleaner air. But how much cleaner do they make the air?

In Wisconsin bars and restaurants, the air is 92% cleaner since the enactment of a July law banning smoking in bars, restaurants and all other enclosed workplaces. Since the ban, just over 3% of the establishments tested have air quality rated unhealthy or worse.

Before the law was passed, only 13% of the establishments had air quality that was rated as satisfactory or good. Now, 97% of these establishments do--results that are every bit as clear as the air now is.

Before the law was passed, over 80% of establishments tested had air quality rated as unhealthy or worse (there are actually two air quality ratings that are unhealthier than unhealthy). Being exposed to secondhand smoke, such as that found in bars and restaurants, is a risk factor for lung cancer.

Air quality was tested in 214 bars and restaurants where smoking was allowed, in 41 counties throughout Wisconsin, between April 2 and June 18, 2010, prior to the implementation of the smoke-free air law. Overall, 21% of them had hazardous air quality, 28% had very unhealthy air quality and 38% had unhealthy air quality.

Measurements were of the amount of fine particulates in the air. Fine particulate matter is defined as particles smaller than 2.5 microns, about one-tenth the width of a human hair. Small particles can penetrate further than large ones can, causing damage deep inside of lung tissue and the walls of arteries.

Air samples were also taken outside of the establishments to ensure that the particulates measured inside were not from polluted air that had infiltrated from outdoors. These samples consistently showed healthy outdoor air with low levels of fine particulate matter.

In all, 183 establishments had air quality that was unhealthy or worse.

After the enactment of the law, tests performed in September and October found only 6 of these 183 establishments had air quality that was unhealthy or worse, and only one had air quality that was hazardous.

Before the law was passed, only 13% of the establishments had air quality that was rated as satisfactory or good. Now, 97% of these establishments do – results that are every bit as clear as the air now is.

The study, conducted by the University of Wisconsin Carbone Cancer system, was released on December 14, 2010.