How many air changes per hour does mold remediation require?

IICRC S520 (Standard for Professional Mold Remediation) recommends a minimum of 4 air changes per hour in remediation work areas. During active disturbance (tearing out moldy materials), higher ACH of 6–8 is recommended to control airborne spore concentrations. For containment setup prior to remediation: establish negative air pressure with at least 4 ACH as a baseline. The negative pressure (−0.02 to −0.03 in. W.G.) prevents contaminated air from migrating to clean areas. HEPA filtration at the air scrubber exhaust captures spores before exhausting air outside the containment.

What size air scrubber do I need for a 1,500 square foot space?

For a 1,500 sq ft space with 9-foot ceilings: volume = 1,500 × 9 = 13,500 cubic feet. At 6 ACH (standard renovation): CFM = (6 × 13,500) / 60 = 1,350 CFM. You'd need two 600–700 CFM air scrubbers running simultaneously, or one 1,500 CFM industrial unit. For basic air quality improvement at 4 ACH: CFM = (4 × 13,500) / 60 = 900 CFM — single large unit or two medium units. Consider ceiling height when sizing — 10-foot ceilings increase volume by 11%, requiring proportionally more CFM.

What is the difference between a negative air machine and an air scrubber?

Functionally, a negative air machine and an air scrubber are the same device — a HEPA-filtered blower. The difference is in how they're used: As a 'negative air machine': air is exhausted outside the containment area to create negative pressure inside the containment zone, preventing contaminated air from leaking out. As an 'air scrubber': air is recirculated within the space through the HEPA filter to clean the room air without pressurizing or depressurizing the space. For mold and asbestos remediation, negative air machine mode is required to protect adjacent areas.

Why is the air scrubber derated below its rated CFM?

Air scrubbers are derated from their rated airflow because rated CFM is measured with a clean filter at zero static pressure. In actual use, several factors reduce effective airflow: filter loading (a partially loaded HEPA adds 0.3–0.5 in. W.C. static pressure, reducing airflow 15–25%); ductwork when running exhaust to outside (each 10 feet of duct adds ~0.1 in. W.C.); elevated dust load in the work area (adds to filter loading faster). A 20% derating factor is standard for estimating working CFM from rated CFM. In heavily contaminated environments, use a 30–40% derating factor and plan for more frequent pre-filter changes.

Air Scrubber Sizing Calculation: CFM, ACH, and Unit Count for Air Filtration Projects

Calculate the required CFM and number of HEPA air scrubber units for any room based on volume and target air changes per hour. Size air scrubbers for mold remediation, construction dust control, and indoor air quality projects.

Related Calculators

Air Scrubber Calculation
Calculate air-scrubber CFM, air changes per hour, unit count, delivered airflow, and treatment cycles.

Air Scrubber Basics

An air scrubber (also called a negative air machine or air filtration device) circulates room air through HEPA and carbon filters to remove particulate matter, mold spores, dust, and VOCs.

Key sizing parameter: Air Changes per Hour (ACH)

ACH = (Scrubber CFM × 60) / Room volume in cubic feet

Required scrubber CFM = (Required ACH × Room volume) / 60

ACH Requirements by Application

Application	Recommended ACH	Notes
General office IAQ improvement	4–6 ACH	Baseline air quality
Dusty construction/renovation	6–8 ACH	Drywall, sanding, demo
Active demolition (asbestos-free)	8–10 ACH	Significant particulate generation
Mold remediation (IICRC S520)	4–6 ACH minimum	Higher during active disturbance
Fire/smoke restoration	6–10 ACH	With carbon filtration
Healthcare/infection control	6–12 ACH	ANSI/ASHRAE 170 requirements
Asbestos abatement containment	4–6 ACH	OSHA-required negative air
Lead paint abatement	4–6 ACH	Per EPA RRP rule requirements

Worked Sizing Example

Room: 20 ft × 30 ft × 9 ft = 5,400 cubic feet

Target: 6 ACH for mold remediation containment

Required CFM = (6 × 5,400) / 60 = 32,400 / 60 = 540 CFM

Select an air scrubber rated for ≥ 540 CFM → choose 500–600 CFM unit or run two 300 CFM units.

Unit count calculation: Unit count = Required CFM / (Unit rated CFM × Derating factor)

For 500 CFM unit at 80% derating: 540 / (500 × 0.80) = 540 / 400 = 1.35 → 2 units

Common Air Scrubber Models and Capacities

Unit Size	Typical CFM	Practical Coverage (6 ACH)
Compact (residential)	150–200 CFM	~1,000–2,000 cu ft
Medium (restoration)	500–600 CFM	~3,000–5,000 cu ft
Large (commercial)	800–1,000 CFM	~5,000–8,000 cu ft
Industrial (2,000+ sq ft)	1,500–2,000 CFM	~10,000–15,000 cu ft

Negative Air Pressure for Containment

For containment work (mold, asbestos, lead): the air scrubber exhausts air outside the containment area to create negative air pressure, preventing contaminants from escaping.

Negative air pressure requirement: - 0.02–0.03 in. W.G. (water gauge) pressure differential - Exhaust the air outside the building or to a HEPA-filtered exhaust port - Replace makeup air through filtered inlets

Filter Types in Air Scrubbers

Filter Stage	Removes	Typical Efficiency
Pre-filter (30–35% MERV)	Large particles, lint	70% efficiency at 10µm
HEPA (MERV 17)	Dust, mold, bacteria	99.97% at 0.3µm
Activated carbon	VOCs, odors, gases	Variable by contaminant
ULPA (optional)	Sub-micron particles	99.999% at 0.12µm

Frequently Asked Questions

How many air changes per hour does mold remediation require?: IICRC S520 (Standard for Professional Mold Remediation) recommends a minimum of 4 air changes per hour in remediation work areas. During active disturbance (tearing out moldy materials), higher ACH of 6–8 is recommended to control airborne spore concentrations. For containment setup prior to remediation: establish negative air pressure with at least 4 ACH as a baseline. The negative pressure (−0.02 to −0.03 in. W.G.) prevents contaminated air from migrating to clean areas. HEPA filtration at the air scrubber exhaust captures spores before exhausting air outside the containment.
What size air scrubber do I need for a 1,500 square foot space?: For a 1,500 sq ft space with 9-foot ceilings: volume = 1,500 × 9 = 13,500 cubic feet. At 6 ACH (standard renovation): CFM = (6 × 13,500) / 60 = 1,350 CFM. You'd need two 600–700 CFM air scrubbers running simultaneously, or one 1,500 CFM industrial unit. For basic air quality improvement at 4 ACH: CFM = (4 × 13,500) / 60 = 900 CFM — single large unit or two medium units. Consider ceiling height when sizing — 10-foot ceilings increase volume by 11%, requiring proportionally more CFM.
What is the difference between a negative air machine and an air scrubber?: Functionally, a negative air machine and an air scrubber are the same device — a HEPA-filtered blower. The difference is in how they're used: As a 'negative air machine': air is exhausted outside the containment area to create negative pressure inside the containment zone, preventing contaminated air from leaking out. As an 'air scrubber': air is recirculated within the space through the HEPA filter to clean the room air without pressurizing or depressurizing the space. For mold and asbestos remediation, negative air machine mode is required to protect adjacent areas.
Why is the air scrubber derated below its rated CFM?: Air scrubbers are derated from their rated airflow because rated CFM is measured with a clean filter at zero static pressure. In actual use, several factors reduce effective airflow: filter loading (a partially loaded HEPA adds 0.3–0.5 in. W.C. static pressure, reducing airflow 15–25%); ductwork when running exhaust to outside (each 10 feet of duct adds ~0.1 in. W.C.); elevated dust load in the work area (adds to filter loading faster). A 20% derating factor is standard for estimating working CFM from rated CFM. In heavily contaminated environments, use a 30–40% derating factor and plan for more frequent pre-filter changes.

Last updated 8/19/2026