NAAQS, Criteria Pollutants, HAPs & AQI

Quick Answer: Know the six criteria pollutants, the difference between primary and secondary NAAQS, HAP major-source thresholds (≥10 tpy single or ≥25 tpy total HAPs), and how AQI categories communicate health risk from ambient monitoring data.

Air quality engineering on the FE Environmental exam begins with the Clean Air Act (CAA) pollutant framework. Before sizing control devices or running dispersion models, you must classify pollutants, identify applicable standards, and understand how ambient data are reported to the public.

Criteria Pollutants and NAAQS

EPA designates criteria pollutants because ambient levels, sources, and health effects justify national standards:

National Ambient Air Quality Standards (NAAQS) include:

• Primary standards — protect public health with adequate margin for sensitive groups (asthmatics, children, elderly).
• Secondary standards — protect welfare: visibility, vegetation, materials, climate-related effects.

States implement State Implementation Plans (SIPs) showing how attainment and maintenance will occur. Designations classify counties as attainment, nonattainment, or unclassifiable for each criteria pollutant.

Exam trap: ground-level vs. stratospheric ozone

Tropospheric ozone is a criteria pollutant formed by NOx + VOC + sunlight. Stratospheric ozone depletion (Montreal Protocol, CFCs) is a separate issue. Exam references to "ozone nonattainment" always mean ground-level smog.

Hazardous Air Pollutants (HAPs)

Section 112 lists 187 HAPs (benzene, formaldehyde, mercury, etc.). Source categories:

MACT (Maximum Achievable Control Technology) sets emission limits based on performance of the best-controlled similar sources. After MACT, EPA may address residual risk if lifetime cancer risk to the maximum exposed individual exceeds policy thresholds (~1 in 10,000 in rulemaking context).

Air Quality Index (AQI)

The AQI converts monitored pollutant concentrations into a uniform 0–500 scale for public communication:

AQI = max sub-index among criteria pollutants reported. Sub-indices use piecewise linear breakpoints tied to NAAQS levels. PM2.5 and ozone drive many high-AQI days in urban areas.

Lead and Other Historical Notes

Lead was a criteria pollutant; ambient lead NAAQS remain though mobile-source lead phased out. Some exam stems still reference lead smelters or aviation gasoline. Greenhouse gases are regulated under separate authority (not criteria NAAQS in the traditional six-pollutant list on older stems — know exam context).

Monitoring and Design Values

Federal Reference Methods (FRM) and Federal Equivalent Methods (FEM) define how ambient data are collected. Design values (e.g., 3-year average of annual 98th percentile PM2.5) compare against NAAQS for attainment determinations.

Prevention of Significant Deterioration (PSD) increments limit how much a pristine area can degrade — separate from NAAQS but part of air planning.

Worked Classification Scenario

A county is in moderate ozone nonattainment. A proposed factory emits 120 tpy NOx as potential to emit (PTE).

• NAAQS context: ozone nonattainment → stricter new-source review than attainment areas.
• Title V: major source threshold often 100 tpy criteria pollutant → likely Title V operating permit required.
• HAPs: if benzene is 2 tpy, not a major HAP source by itself; evaluate combined HAP total separately.

Mobile, Area, and Point Sources

Emission inventories sum mass per time by source category before modeling or permitting.

FE Exam Focus

Memorize criteria pollutant names, primary vs. secondary NAAQS purpose, HAP major thresholds, AQI category breakpoints conceptually, and attainment vs. nonattainment consequences for permitting. Handbook tables supply specific numbers — practice locating pollutant-specific Significant Emission Rates (SER) for modifications.

Exam trap: Confusing emission standards (source limits in lb/MMBtu or tpy) with ambient NAAQS (µg/m³ or ppm in outside air). A source can meet its permit while a region remains in nonattainment.

Solid pollutant taxonomy and AQI literacy unlock every subsequent air chapter topic — dispersion, controls, and permitting.

State Implementation and SIP Basics

Each state adopts State Implementation Plans (SIPs) showing how attainment and maintenance of NAAQS will occur. SIPs include emission inventories, control measures, and monitoring networks. When EPA disapproves a SIP element, federal implementation plans may apply. Transportation conformity requires that federally funded highway and transit projects not worsen air quality in nonattainment areas — linking NAAQS status to infrastructure funding.

Exceptional events (wildfires, dust storms) may be excluded from attainment determinations if documented — policy context for interpreting monitored exceedances on exam scenarios.

Regional Haze and Secondary Standards

Regional Haze rules protect Class I area visibility — related to secondary NAAQS welfare protection. While detailed haze modeling is beyond FE scope, know that secondary standards address welfare effects (vegetation, materials, visibility) distinct from primary health-based limits. Exam items may ask whether a standard is primary or secondary when discussing crop damage versus respiratory hospital admissions.