15.2 Environmental Quality Standards: Drinking Water MCLs & Air Quality Standards (NAAQS)

Once the receptor is no longer an 8-hour worker, the OEL toolkit is the wrong default. Community air, public drinking water, Superfund soil and groundwater, stack emissions, and chemical-release emergencies each have their own legal instruments. Domain III.D.4 expects you to know which instrument applies, whether it is health-only or feasibility-modified, and whether it is a chronic quality standard or an acute incident value. Copying an OSHA PEL into a residential scenario, or copying a NAAQS into a 15-minute release, is the classic error.

Numeric tables for NAAQS and MCLs change when EPA revises the underlying rules. Treat any µg/L or ppm figure below as an illustrative example and confirm the live EPA/CFR table when you practice with current regulations. The DABT-stable facts are the architecture: MCL versus MCLG, criteria pollutants, Superfund risk range, emission-limit families, AEGLs, and PALs.

Drinking water: MCLG versus MCL

The Safe Drinking Water Act (SDWA) authorizes National Primary Drinking Water Regulations for public water systems. Two numbers travel together:

• Maximum Contaminant Level Goal (MCLG) — a nonenforceable health goal set at a level at which no known or anticipated adverse effect occurs, with an adequate margin of safety. Feasibility is not part of the MCLG. For contaminants treated as linear, no-threshold carcinogens, EPA has often set the MCLG at zero, because the Agency does not identify a nonzero "safe" dose under that default. Noncancer MCLGs are typically built from an RfD (or similar) plus drinking-water exposure assumptions.
• Maximum Contaminant Level (MCL) — the enforceable limit delivered to users of a public water system. The SDWA directs EPA to set the MCL as close to the MCLG as feasible, using best available treatment technology and considering the ability to measure the contaminant (practical quantitation). When the MCLG is zero, the MCL is usually a nonzero number limited by treatment, analytical chemistry, or both.

Illustrative examples (verify current 40 CFR Part 141 / EPA drinking-water tables; they are not a frozen exam crib sheet): EPA set the arsenic MCL at 0.010 mg/L (10 µg/L) with an MCLG of 0. Benzene has an MCLG of 0 and an MCL of 0.005 mg/L. Lead is a teaching trap: the MCLG is 0, but lead is managed as a treatment technique under the Lead and Copper Rule rather than a simple MCL at the tap. Acrylamide and epichlorohydrin likewise have MCLGs of 0 and are controlled as treatment techniques (limits on monomer in treatment polymers) because standardized tap-water methods were not the chosen enforcement path.

When treatment to an MCL is not practical, EPA may require a treatment technique (TT) instead of a numerical MCL. Disinfectants have maximum residual disinfectant levels (MRDLs) and goals (MRDLGs), which use the same health-goal versus enforceable-limit logic. Secondary MCLs (taste, odor, staining) are nonenforceable welfare guidelines, not primary health standards.

MCLs are not automatically residential tap goals for private wells, and they are not worker OELs. They do often become applicable or relevant and appropriate requirements (ARARs) for Superfund groundwater, which is why the same chemical can show up as an MCL in a Record of Decision and as a PEL in an occupational stem.

Ambient air: NAAQS criteria pollutants

The Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for six criteria pollutants:

1. Ground-level ozone (photochemical oxidants)
2. Particulate matter (PM2.5 and PM10 are regulated as distinct indicators)
3. Carbon monoxide (CO)
4. Sulfur dioxide (SO2) (sulfur oxides)
5. Nitrogen dioxide (NO2) (nitrogen oxides)
6. Lead

They are called "criteria" pollutants because EPA must publish air-quality criteria documents characterizing health and welfare effects, then set standards. Primary NAAQS protect public health, including susceptible groups, with an adequate margin of safety. Secondary NAAQS protect public welfare — visibility, crops, materials, animals, and climate-related welfare effects. The Act calls for periodic scientific review; EPA has tightened several primary standards in recent decades (ozone and PM2.5 are the usual news examples). Do not memorize a 2026 screenshot of ppm or µg/m³ levels as the official DABT number set. Items test whether you know the six pollutants, primary versus secondary, and that NAAQS are outdoor ambient standards implemented through state implementation plans — not 8-hour workplace PELs and not stack emission limits.

Hazardous air pollutants (HAPs) are a different list under Clean Air Act section 112. They are controlled mainly through emission standards, not through NAAQS.

Emission limits: from ambient goals to the stack

Ambient standards do not, by themselves, tell a facility how many pounds it may emit. Emission limits are the technology- and residual-risk rules that sit between the plant and the NAAQS or HAP goal:

• New Source Performance Standards (NSPS), Clean Air Act section 111 — technology-based limits for new or modified stationary sources of criteria and other pollutants.
• National Emission Standards for Hazardous Air Pollutants (NESHAP) / MACT (maximum achievable control technology), section 112 — technology-based HAP limits, often followed by a residual-risk review that can add further health-based cuts.
• State implementation plan (SIP) rules — the local machinery that makes NAAQS attainment real (permits, offsets, reasonably available control technology).
• Water analogs: NPDES effluent limits and effluent guidelines under the Clean Water Act; waste analogs under RCRA.

Emission limits are often feasibility-shaped even when the underlying health goal is not. That is the same MCL-versus-MCLG logic in a different medium: health science sets the direction; engineering and cost often set the enforceable number.

Superfund cleanup goals and the risk range

CERCLA/Superfund cleanups follow the National Contingency Plan (NCP), 40 CFR Part 300. For known or suspected carcinogens, EPA generally manages excess lifetime cancer risk in the range of about 1 × 10^-6 to 1 × 10^-4 (one in a million to one in ten thousand). The 10^-6 end is commonly the point of departure when risk-based preliminary remediation goals are developed; risks approaching or above 10^-4 usually pull the decision toward action. For noncancer effects, the usual comparison point is a hazard index of 1. These are risk-management policy choices, not laws of toxicology: a 10^-5 residual risk can be acceptable at one site and not at another once ARARs, technical limitations, detection limits, and land use are weighed.

Where an MCL exists, it often becomes an ARAR for groundwater used as drinking water, so a Superfund goal may look like an MCL even though the NCP risk range is the backstop when ARARs do not finish the job. Do not use OSHA PELs as residential soil PRGs, and do not use AEGLs as 30-year groundwater goals.

Acute and incident values: AEGLs and EPA PALs (III.D.4 C)

Chronic MCLs and NAAQS are the wrong tools for a tank-car rupture at 2 a.m. Two EPA-linked incident systems appear on the ABT outline:

Acute Exposure Guideline Levels (AEGLs) are airborne concentrations for rare, usually accidental releases. They are designed for the general public, including susceptible individuals (children, elderly people, and others). Three severity levels are developed for each of five short durations: 10 minutes, 30 minutes, 1 hour, 4 hours, and 8 hours:

• AEGL-1 — above this concentration, notable discomfort, irritation, or certain asymptomatic nonsensory effects are predicted. Effects are not disabling and are transient and reversible when exposure stops.
• AEGL-2 — above this concentration, irreversible or other serious, long-lasting adverse effects or impaired ability to escape are predicted. This is the usual planning number for evacuation and shelter-in-place thresholds.
• AEGL-3 — above this concentration, life-threatening effects or death are predicted.

Concentrations below AEGL-1 can still produce mild odor or sensory irritation. AEGLs are not OSHA PELs, not NAAQS, and not 70-year cancer-risk concentrations. AIHA Emergency Response Planning Guidelines (ERPGs) are a related 1-hour community-planning set; AEGLs differ by covering five durations and by explicitly including susceptible people.

EPA Provisional Advisory Levels (PALs) are health-based advisory concentrations for homeland-security and other incidents. They are not promulgated as regulations and are not formal regulatory guidance. Risk managers may use them for evacuation, temporary re-entry, and resumed use of infrastructure such as water systems. Three severity levels — PAL 1 (mild, transient, reversible effects), PAL 2 (serious, possibly irreversible effects), and PAL 3 (life-threatening effects or death) — are developed for 24-hour, 30-day, 90-day, and 2-year durations, for both ambient air and drinking water. That media-and-duration span is the PAL discriminator: AEGLs are hours-scale air values; PALs extend into days-to-years and into water.

Putting the right number on the right scenario

• Worker, ordinary shift — PEL as the legal floor; TLV/REL as health context; hierarchy of controls to meet them.
• Worker, respirator failure in a high concentration — IDLH / OSHA IDLH-atmosphere rules.
• Community, ordinary outdoor air — NAAQS; SIPs and emission limits to attain them.
• Community, public water — MCLG as the health goal; MCL or TT as the enforceable rule.
• Abandoned site, 30-year resident — Superfund risk range, HI of 1, and ARARs (often MCLs for groundwater).
• Railcar, 1-hour plume — 1-hour AEGL-2 for irreversible/escape-impairing effects; AEGL-3 for life threat.
• Contaminated water system after an attack or spill, days to months — EPA PALs for water (and air) at 24 h / 30 d / 90 d / 2 y.

Exam traps

• MCLG of 0 is not an MCL of 0. Enforceable MCLs usually sit above a zero goal because of feasibility.
• NAAQS are not HAPs and not PELs. Six criteria pollutants; primary versus secondary; outdoor ambient air.
• Do not recite last year's NAAQS or MCL table as if ABT required that snapshot. Cite structure and, if you mention a number, label it as an EPA example that can be revised.
• Superfund 10^-6 is a point of departure, not a cosmic zero. The NCP range runs up to about 10^-4.
• AEGL ≠ PAL ≠ IDLH ≠ PEL. Duration, receptor (worker vs public), medium (air vs water), and legal status all differ.

Key takeaways

• MCLGs are health goals; MCLs (and treatment techniques) are enforceable and feasibility-constrained.
• NAAQS cover six criteria pollutants with primary health and secondary welfare standards that EPA can revise.
• Superfund generally uses a 10^-6 to 10^-4 excess-cancer-risk range and a noncancer HI of 1.
• AEGLs (10 min to 8 h, air, public) and EPA PALs (24 h to 2 y, air and water, advisory) are the incident values in III.D.4 C.