Emission rate (g/hr)
The emission rate is the grams of particulate matter (smoke) a stove releases per hour of operation, measured over a standardized EPA lab test. The 2020 NSPS caps new room heaters at 2.0 g/hr (2.5 g/hr when tested with cord wood); the cleanest certified stoves test below 1 g/hr.
Particulates — fine soot and condensed organics — are the pollutant that makes wood smoke a health issue, so the certified g/hr figure is the headline number in the EPA database. It is a weighted average across burn rates, not a worst-case or best-case figure.
The lab number assumes dry test fuel and correct operation. A certified 1.5 g/hr stove burning unseasoned wood with the air choked down can emit many times its rating — fuel moisture and operation matter as much as the design.