What do the emissions limits of 4.5g and 7.5g pertain to in NSPS?

The emissions limits of 4.5 grams and 7.5 grams refer to the particulate matter emissions from wood-burning appliances, specifically in the context of the New Source Performance Standards (NSPS). These limits apply primarily to different types of wood stoves, where the 4.5 grams per hour limit pertains to certified emissions for the cleaner, catalytic wood stoves, while the 7.5 grams per hour limit is applicable to non-catalytic wood stoves.

Catalytic devices are designed to reduce emissions by combusting gases that would otherwise escape through the flue, thus allowing for a lower emissions limit. Non-catalytic devices, while they also produce heat, typically have higher emissions because they do not utilize the same technology for complete combustion. The distinction between the two types is critical in understanding regulatory compliance as outlined by the EPA under NSPS guidelines.

The other choices relate to areas not directly connected to these specific emissions limits for wood-burning stoves, making them less relevant to the question posed.