NSR applicability is the gateway question for any new construction or modification project at an existing facility. Get the answer wrong in either direction and the consequences are significant. A false negative means construction without required permits. A false positive means unnecessary permitting costs and delays. Both outcomes are avoidable with a careful analysis.
The mistakes that produce wrong answers tend to fall into a handful of categories. This article covers the most common ones.
Confusing actual emissions with potential-to-emit
NSR for major modifications uses actual-to-projected-actual emissions for existing units and potential-to-emit for new units. The distinction matters enormously. A facility that has been running at 40 percent capacity for five years has actual emissions well below its potential-to-emit. Adding new equipment may produce a significant actual emissions increase even if the project does not increase the facility's theoretical potential.
The baseline for actual emissions is typically the average of the two highest years of actual emissions in the ten years preceding the project. Choosing the wrong baseline years, or using potential-to-emit where actual emissions should be used, produces a fundamentally incorrect analysis.
Projected actual emissions: For the modified unit, the analysis uses projected actual emissions after the modification, not potential-to-emit. Projected actual reflects how the unit is reasonably expected to operate, accounting for the demand on the modified unit and any enforceable limits on hours or throughput. If the projection is not supportable, the agency will substitute potential-to-emit.
Getting the significance thresholds wrong
NSR applicability requires that a net emissions increase be significant. Significance thresholds differ by pollutant and by area type. The most common error is applying attainment area thresholds to a nonattainment area or vice versa.
| Pollutant | PSD Significance (attainment) | Nonattainment NSR trigger |
|---|---|---|
| NOx | 40 tons/year | Varies by area classification |
| SO2 | 40 tons/year | Varies by area classification |
| CO | 100 tons/year | 100 tons/year (serious areas) |
| PM2.5 | 10 tons/year | 10 tons/year |
| PM10 | 15 tons/year | 15 tons/year |
| VOC | 40 tons/year | Varies by area classification |
Facilities in ozone nonattainment areas face lower significance thresholds for VOC and NOx. A project that falls below the PSD significance threshold for NOx may still trigger nonattainment NSR if the area is classified as serious or above for ozone.
Failing to account for netting
Netting allows a facility to offset the emissions increase from a project with contemporaneous emissions decreases elsewhere at the facility. If the net increase after netting falls below the significance threshold, major NSR is not triggered.
The most common mistakes in netting calculations involve the contemporaneous period and the type of reductions that qualify.
Contemporaneous period: Netting credits must come from emissions changes that occurred within a specific lookback period typically five years before construction begins. Changes outside that window do not qualify. A unit that was shut down six years ago does not provide netting credit.
What counts as a decrease: Only actual decreases in actual emissions count for netting purposes. A decrease in potential-to-emit without a corresponding decrease in actual emissions does not qualify. Reductions from units that are being shut down as part of the same project can count, but the shutdown must be real and permanent.
Voluntary reductions do not automatically generate credits: Reducing emissions voluntarily before a project begins in order to generate netting credits is a recognized strategy, but it requires careful timing and documentation. The reductions must be real, quantifiable, and enforceable. Agencies scrutinize pre-project reductions intended to avoid NSR review.
Misidentifying the emission unit for modification analysis
NSR applies to modifications to existing units. Correctly identifying which unit is being modified affects the baseline emissions calculation. Adding capacity to an existing unit is different from adding a new unit to an existing facility. The distinction determines whether the analysis compares actual-to-projected-actual for an existing unit or uses potential-to-emit for a new unit.
Physical changes and changes in method of operation are both modifications. Changing fuel type, increasing production rate, extending operating hours, or removing an operational limit are all potentially triggering modifications even if no new physical equipment is installed.
Overlooking applicable exemptions
Several categories of physical changes and operational changes are exempt from NSR by regulation. These include routine maintenance, repair, and replacement; increases in hours of operation within existing permits; and certain fuel switching scenarios. Failing to check whether an exemption applies can lead to unnecessary permitting. Incorrectly claiming an exemption that does not apply is worse.
The routine maintenance exemption is the most frequently misapplied. A project qualifies as routine maintenance only if it is of the type carried out routinely at comparable facilities and does not extend the useful life of the unit or change its design capacity. Replacing a boiler with a newer, more efficient model is not routine maintenance. Replacing a worn burner assembly on an existing boiler with an identical replacement likely is.
Not documenting the analysis contemporaneously
NSR applicability analyses are reviewed at the time of permit application, during inspections, and sometimes years later during enforcement actions. An analysis that was done informally at the time of the project and reconstructed after the fact is not defensible. The documentation needs to be contemporaneous: baseline emissions data, the specific project description, the emissions calculations, the netting analysis if applicable, and the conclusions.
For projects that fall just below a significance threshold, the documentation is particularly important. An agency that reviews the project and finds the numbers marginal will look at how the analysis was done and when it was documented.
How AirComply handles NSR applicability
AirComply calculates potential-to-emit from source parameters, applies the correct significance thresholds for the facility's location and area classification, runs netting calculations against baseline actual emissions, and identifies which NSR pathway applies. The analysis is documented in a structured output the consultant can review and use as the basis for the formal applicability determination. Every regulatory citation is retrieved live from the current CFR, not from memory or outdated guidance.
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AirComply calculates PTE, applies significance thresholds, and runs netting with live CFR retrieval.
Request demo accessThe bottom line
NSR applicability is not a box to check. It is a legal determination with significant consequences if wrong. The most common mistakes are not conceptual errors in understanding the framework. They are calculation errors, baseline selection errors, and documentation failures that result from doing the analysis quickly without systematic rigor.
A careful analysis takes time but produces a defensible result. An informal analysis done under deadline pressure produces risk that surfaces at the worst possible moment: during an inspection, a permit renewal, or an enforcement action years after the project was built.