Air Changes per Hour (ACH): Ventilation Performance Metric

Air changes per hour (ACH) specifies how many complete volumes of air are introduced to or removed from a space in sixty minutes. The metric is expressed as ACH = Q / V, where Q is volumetric airflow in cubic metres per hour and V is the space volume in cubic metres. Because ACH combines dimensional measurements, airflow testing, and ventilation effectiveness factors, it has become the common language linking infection control, comfort, and energy targets across building types.

This article formalises the ACH definition, documents its historical evolution, examines contemporary measurement methods, and explores how the metric informs design and operations. Along the way you will find references to complementary explainers such as the pressure unit guide and interactive tools like the ACH calculator that support commissioning workflows.

Definition and Fundamental Relationships

Deriving ACH from primary measurements

To compute ACH you measure the volumetric airflow delivered to a zone and divide it by the zone volume. In practice, volumetric flow is the product of duct cross-section area and average air velocity, both of which must be recorded using calibrated instrumentation. Balometers, thermal anemometers, and Pitot-static traverses yield flow data which are adjusted for density variations using the standard atmosphere assumptions or project-specific barometric readings. Zone volume requires traceable length measurements, an aspect addressed in the length metrology overview.

Ventilation effectiveness and equivalent ACH

Air supplied to an occupied zone may not mix uniformly. Standards therefore introduce a ventilation effectiveness factor, often written as epsilon_v, that converts an apparent ACH into an equivalent value describing contaminant removal. Displacement ventilation may achieve epsilon_v values greater than one because clean air is targeted to breathing zones, while ceiling-mixing systems typically apply values between 0.8 and 1.0. Infection control guidelines additionally recognise clean air delivery rate (CADR), which integrates filtration, ultraviolet germicidal irradiation, and outdoor air dilution into an ACH-equivalent benchmark for comparing mitigation strategies.

Historical Development of the Metric

From natural ventilation heuristics to mechanical standards

The earliest references to air change targets arose in nineteenth-century hospital design manuals that advocated large window-to-floor ratios to mitigate airborne disease. Engineers such as John Shaw Billings and Florence Nightingale translated these heuristics into recommended air volumes per occupant, effectively foreshadowing modern ACH tables. With the advent of electric fans and ducted supply in the early twentieth century, airflow could be measured directly, enabling health agencies to codify minimum air changes for schools, barracks, and industrial buildings.

Modern harmonisation and pandemic responses

By the mid-twentieth century, organisations including ASHRAE, CIBSE, and DIN had issued standards detailing ACH values for diverse occupancies. The energy crises of the 1970s prompted reductions in ventilation rates, but subsequent research on sick building syndrome reinstated higher thresholds. The 2003 SARS outbreak and the COVID-19 pandemic accelerated global harmonisation efforts that now cross-reference ACH with infection probability models and real-time indoor air quality monitoring. Contemporary guidance couples ACH targets with performance verification, requiring commissioning agents to validate both airflow and contaminant decay rates.

Measurement Techniques and Modelling Concepts

Tracer gas decay and particulate clearance tests

To verify ACH in existing buildings, practitioners frequently perform tracer gas decay tests. Carbon dioxide, sulfur hexafluoride, or nitrous oxide is released until the space is well mixed, after which concentration decay is logged. The slope of the semi-log concentration curve yields the effective air change rate, naturally accounting for infiltration and exfiltration. Particle decay tests using neutralised aerosols provide similar insight where chemical tracers are impractical, particularly in healthcare settings that monitor airborne pathogens.

Continuous monitoring and analytics

Modern building automation systems integrate airflow stations, differential pressure sensors, and indoor air quality monitors to infer ACH continuously. Bayesian data fusion combines these inputs with digital twins, ensuring that calculated ACH aligns with occupancy schedules and demand-controlled ventilation sequences. The server room calculator exemplifies how ventilation data inform thermal load management in high-density spaces.

Modelling the energy implications

Raising ACH increases heating and cooling loads because outdoor air must be conditioned to indoor set-points. Energy models use psychrometric relationships and heat balance equations to quantify the penalty, after which designers evaluate mitigation strategies such as energy recovery ventilators, demand control algorithms, and thermal storage. Pairing ACH calculations with the HRV sizing tool highlights how sensible and latent energy recovery can offset higher ventilation rates without compromising comfort.

Applications Across Sectors

Healthcare, laboratories, and cleanrooms

Healthcare ventilation codes prescribe ACH values ranging from six for general patient rooms to twenty-five or more for airborne infection isolation rooms. Laboratories balance ACH with fume hood containment, chemical storage, and energy efficiency, often using variable air volume controls to maintain target air changes when hoods close. Cleanrooms integrate ACH targets with particle count requirements, a relationship quantified through ISO 14644 classification tables and our cleanroom calculator.

Education, residential, and commercial buildings

Schools and offices rely on ACH benchmarks to control carbon dioxide concentration, odours, and occupant comfort. Demand-controlled ventilation strategies modulate outdoor air supply while upholding minimum ACH thresholds during occupancy. Residential buildings increasingly adopt balanced ventilation systems with heat or energy recovery to maintain ACH between 0.3 and 0.5 without introducing drafts or excessive humidity swings. Commissioning plans therefore include ACH measurement as a key performance indicator linked to warranty requirements and indoor environmental quality certifications.

Emergency response and temporary installations

Temporary healthcare facilities, field hospitals, and mobile laboratories must rapidly configure ventilation systems to meet ACH mandates. Portable HEPA filtration units and ultraviolet disinfection devices are rated by their equivalent ACH contribution, allowing planners to verify that minimum dilution levels are achieved even when permanent ductwork is unavailable. Similar considerations apply to emergency shelters and isolation rooms used during wildfire events, where ACH must balance contaminant removal with particulate infiltration.

Importance and Emerging Directions

Public health significance

ACH directly influences airborne transmission risk by affecting contaminant residence time. Epidemiological models such as Wells-Riley and dose-response frameworks use ACH as a primary parameter, demonstrating that doubling ACH can halve infection probability under otherwise identical conditions. Consequently, ventilation upgrades often form the first line of defence against respiratory outbreaks in schools, transit hubs, and workplaces.

Toward outcome-based performance metrics

Future practice is shifting from prescriptive ACH targets toward outcome-based performance contracts that verify actual dilution effectiveness. Integrating sensor networks, occupant feedback, and real-time analytics will allow ACH to be coupled with pollutant exposure, comfort indices, and energy use intensity. By situating ACH within a broader context of resilience and sustainability, building professionals can optimise ventilation strategies that support both health and decarbonisation objectives.