Many of the studies and reports I've seen indicate approximately 1.5 tons of PM per heating season. I suppose some states simply are trying to stay ahead of the game by not letting the devices accumulate. When you think about some Midwestern states and the thousands of OWBs installed, 1.5 tons each does impact the breathing envelope, especially where there is an accumulation of the devices. Although many state DEQs know this they are constrained by state politics. However, some have covered their bases by listing OWBs as a potential threat in their implementation plans just in case somewhere down the line federal air standards are exceeded and tier 1 problems alone can't fix the issue.