HVAC Air Quality Terms and Definitions Glossary

This page provides standardized definitions for terms used across HVAC system design, indoor air quality (IAQ) assessment, filtration specification, and regulatory compliance. The glossary covers core vocabulary drawn from ASHRAE standards, EPA guidance, and OSHA regulatory frameworks. Precise terminology is essential because misapplication of a single term — such as confusing ACH (air changes per hour) with OA (outdoor air) fraction — can produce code violations, failed inspections, or occupant health consequences in residential, commercial, and institutional settings.

Definition and scope

This glossary addresses terminology specific to the intersection of HVAC mechanical systems and indoor air quality pollutants. Terms are drawn from named standards bodies including ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), the U.S. Environmental Protection Agency (EPA), the National Institute for Occupational Safety and Health (NIOSH), and the American National Standards Institute (ANSI).

Core term categories covered:

The glossary is intended for facility managers, HVAC technicians, building code officials, and IAQ consultants. It does not substitute for the full text of any cited standard or regulatory document.

How it works

A–C

ACH (Air Changes per Hour): A measure of how many times the total air volume of a space is replaced within one hour. Calculated as total airflow (CFM) × 60 ÷ room volume (cubic feet). ASHRAE Standard 62.1-2022 specifies minimum ventilation rates that translate into minimum ACH values for occupancy categories (ASHRAE 62.1).

CADR (Clean Air Delivery Rate): A metric established by AHAM (Association of Home Appliance Manufacturers) measuring the volume of filtered air delivered per minute by a portable air cleaner, rated separately for smoke (0.09–1.0 µm particles), dust (0.5–3.0 µm), and pollen (5.0–11.0 µm).

CFM (Cubic Feet per Minute): The standard volumetric airflow unit in U.S. HVAC design. Minimum supply CFM per occupant or per square foot is mandated in ASHRAE 62.1-2022 tables for non-residential applications and ASHRAE 62.2-2022 for residential construction.

CO₂ (Carbon Dioxide): A surrogate IAQ indicator for occupancy-driven ventilation dilution. The EPA and ASHRAE do not set a regulatory limit for indoor CO₂, but readings above 1,100 ppm above outdoor ambient levels indicate inadequate outdoor air delivery. See carbon dioxide monitoring in HVAC for sensor placement and threshold protocols.

CO (Carbon Monoxide): A colorless, odorless combustion byproduct. OSHA's permissible exposure limit (PEL) for CO is 50 ppm as an 8-hour time-weighted average (OSHA 1910.1000). HVAC systems are both a pathway for CO infiltration and a mitigation vector; see carbon monoxide HVAC safety.

D–H

Dew Point: The temperature at which air becomes saturated and water vapor condenses. HVAC humidity control targets a dew point range that keeps relative humidity between 30% and 60% indoors, per ASHRAE Standard 55-2020, to suppress mold growth and occupant discomfort. See HVAC humidity control and air quality.

ERV (Energy Recovery Ventilator): A mechanical ventilation device that exchanges both sensible heat and moisture (latent heat) between exhaust and supply airstreams. ERVs are distinguished from HRVs (Heat Recovery Ventilators), which transfer sensible heat only. In humid climates, ERVs reduce the dehumidification load imposed by outdoor air intake.

HEPA (High-Efficiency Particulate Air): A filter classification requiring capture of ≥ 99.97% of particles at 0.3 µm diameter, per DOE STD-3020-2015. HEPA filters are not interchangeable with MERV-rated media; they require fan systems engineered for their higher static pressure drop.

M–R

MERV (Minimum Efficiency Reporting Value): An ASHRAE 52.2 filter rating scale from MERV 1 (lowest efficiency) to MERV 20 (highest). MERV 13 filters capture ≥ 85% of particles in the 1.0–3.0 µm range and are recommended by the EPA and CDC for reducing airborne pathogen exposure in occupied buildings. Full rating breakdowns are covered at MERV ratings explained.

MVOC (Microbial Volatile Organic Compound): A subset of VOCs produced by fungal or bacterial metabolism. MVOCs are associated with damp building materials and are a recognized indicator of mold colonization in duct systems and air handlers. See mold prevention and HVAC air quality.

OA Fraction (Outdoor Air Fraction): The ratio of outdoor air supplied to total supply air delivered to a space. ASHRAE 62.1-2022 mandates a minimum OA fraction calculated through the Multiple Zone Recirculating System procedure for VAV systems.

PM2.5 / PM10: EPA size classifications for particulate matter. PM2.5 refers to particles with an aerodynamic diameter ≤ 2.5 µm; PM10 includes particles ≤ 10 µm. The EPA National Ambient Air Quality Standard (NAAQS) sets the annual PM2.5 standard at 9.0 µg/m³ (revised in 2024 from the prior 12.0 µg/m³ limit) (EPA NAAQS Table).

Radon: A radioactive, naturally occurring gas (EPA Action Level: 4 pCi/L) that accumulates in below-grade spaces. HVAC systems interact with radon through pressure differentials and sub-slab depressurization mitigation strategies. See radon mitigation and HVAC systems.

S–V

SRE (Sensible Recovery Efficiency): The percentage of sensible (temperature-based) heat recovered by an HRV or ERV under AHRI Standard 1060 test conditions.

VOC (Volatile Organic Compound): Carbon-based gases emitted at room temperature from building materials, cleaning products, and combustion. EPA classifies VOCs broadly; ASHRAE Standard 62.1-2022 addresses minimum ventilation as a dilution strategy. HVAC-specific mitigation approaches are detailed at volatile organic compounds and HVAC mitigation.

Common scenarios

Glossary terms appear in distinct professional contexts, and the operational stakes differ by setting:

  1. Permit and plan review: Building officials referencing ASHRAE 62.1-2022 OA rates or MERV filter specifications on mechanical permit drawings require precise term alignment; discrepancies between "design CFM" and "minimum OA CFM" are a documented source of plan check rejections.
  2. IAQ investigation: NIOSH Health Hazard Evaluation reports routinely distinguish between PM2.5 concentrations attributable to HVAC recirculation versus outdoor infiltration — a distinction requiring accurate use of CADR, ACH, and OA fraction terms.
  3. Commissioning and balancing: TAB (Testing, Adjusting, and Balancing) technicians certified under NEBB or AABC protocols apply ACH, CFM, and static pressure terminology as primary measurement categories.
  4. Wildfire smoke response: During wildfire events, MERV and CADR ratings become operationally critical for facilities managers determining whether existing filtration can handle elevated PM2.5 loads. Guidance specific to this scenario is covered at HVAC air quality and wildfire smoke.
  5. Healthcare and school IAQ compliance: ASHRAE Standard 170-2021 governs ventilation in healthcare facilities and specifies ACH minimums by room type; schools reference ASHRAE 62.1-2022 and CDC guidance post-2020. See HVAC air quality in schools and healthcare.

Decision boundaries

Accurate term selection requires distinguishing between terms that are frequently conflated:

MERV vs. HEPA: MERV is a gradient efficiency scale applied to panel, pleated, and bag filters in standard HVAC airstreams. HEPA is a pass/fail absolute efficiency standard requiring ≥ 99.97% capture at 0.3 µm. HEPA is not a MERV rating; the two classifications exist under different test standards (ASHRAE 52.2 vs. DOE STD-3020-2015) and cannot be cross-substituted without fan and duct system redesign.

ERV vs. HRV: Both devices recover energy from exhaust air. The boundary is moisture transfer: ERVs transfer water vapor (latent energy), making them appropriate in mixed-humid and hot-humid climate zones. HRVs transfer sensible heat only and are preferred in cold-dry climates where interior moisture retention is des

References

📜 6 regulatory citations referenced  ·  ✅ Citations verified Mar 01, 2026  ·  View update log

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