A new report from by the Sierra Club estimates how much sulfur dioxide is emitted from nine Kentucky power plants and it finds that all nine of them—including the Mill Creek and Cane Run power plants in Louisville—are violating the national air quality standards.

Sulfur dioxide has been linked to health issues like pulmonary inflammation, asthma, emphysema and other lung conditions.

But the data in the Sierra Club’s report is collected differently from how the local Air Pollution Control District measures sulfur dioxide. Here are three things to know when reading the report, or perusing Metro Government’s air monitoring data:

  1. The EPA set the 1 hour sulfur dioxide standard at 75 parts per billion in August, 2010. That means since then, air monitoring has to show that there’s less than 75 units of sulfur dioxide in every billion parts of air measured. The standard relies on an hourly average.
  2. The Air Pollution Control District uses air monitoring to keep track of the pollution, while the Sierra Club used air modeling. Air monitoring takes a snapshot of the air at any given moment, while modeling relies on lots of different known and estimated factors (like weather, the height of smokestacks, etc.) to make a fairly educated prediction about how much pollution is present at any given moment. Both are accepted methods—the EPA has approved the city’s use of monitoring, and has also endorsed modeling.
  3. The Sierra Club’s data shows egregious air pollution violations. At Cane Run, it models sulfur dioxide emissions at 27 times the allowable amount. At Mill Creek, it estimates levels about six times too high. But the actual monitoring data from the city tells a different story. It shows that in 2012, there have been only 13 exceedences for sulfur dioxide, all at the Watson Lane Elementary monitor, which is only about a mile away from Mill Creek. The highest daily maximum recorded there is about three times higher than the air quality standard.

So there’s differing data, and whatever you look at, sulfur dioxide is definitely a concern in Jefferson County. But I think the takeaway from this study and the city’s data is that the EPA could revisit the sulfur dioxide air quality standard in 2015 (or even sooner if there's a compelling reason to). If it does, and if it makes it more stringent, Louisville will have to find a way to come into compliance (the area is technically listed as in attainment for sulfur dioxide, despite several exceedences).

This should be easier over the next few years. Coal-fired power plants are the main source of sulfur dioxide, and the Cane Run Power Plant is scheduled to be retired by 2016 and replaced by natural gas. And at Mill Creek, Louisville Gas & Electric is in the process of installing new pollution controls which will reduce the amount of sulfur dioxide the plant emits. There’s also the Gallagher plant in southern Indiana, which has been blowing pollution into Louisville for fifty years. Half of the plant was retired earlier this year, which should also help reduce Louisville’s air pollution.