California is always on the cutting edge of environmental regulations, so it's no surprise that several of its air quality management districts have recently mandated lower nitrous oxide (NOx) emissions for large boilers.
The reason for the new regulations is that NOx is a precursor to ozone, and several areas in the state have not improved their ground-level ozone quality as quickly as the federal government requires. According to the EPA, inhaling ground-level ozone can result in health effects in broad segments of the population, including respiratory symptoms (e.g., coughing, throat irritation, shortness of breath), reduced lung function, and airway inflammation.