NSPS 2020 rule (Step 2)
The NSPS (New Source Performance Standards) for residential wood heaters is the EPA regulation capping particulate emissions from new stoves. Its "Step 2" limits, in force since May 2020, cap room heaters at 2.0 g/hr (2.5 g/hr cord-wood tested) — only compliant models may be manufactured or sold new in the US.
The 2015 rule introduced Step 1 limits (4.5 g/hr) and the 2020 Step 2 limits roughly halved them, also bringing forced-air furnaces and hydronic heaters under emission caps for the first time. That regulatory step is why the current certified list is dramatically cleaner than the installed base.
NSPS limits apply to manufacture and sale of new heaters, not to stoves already installed — an old stove remains legal to operate in most (not all) jurisdictions, though several states and air districts restrict or incentivize replacing pre-certification units.