Ventilation & Airflow Testing in Factories: ACH & Air Velocity Guide
🌱 Last updated: September 1, 2026
Ventilation and Airflow Testing in Factories: Air Velocity, Airflow, ACH, Methods & Requirements
A factory can have exhaust fans running, open windows and fresh-air ducts installed-and still have poor ventilation in the areas where people actually work.
This happens because ventilation is not simply about whether a fan is present.
The real questions are:
How fast is the air moving?
How much air is actually entering or leaving the area?
Is fresh air reaching the occupied work zone?
Is exhaust air being removed effectively?
Are there stagnant areas?
How many times is the room air being replaced?
Has a change in machinery, storage or layout affected the original airflow pattern?
These questions are answered through ventilation and airflow testing.
Industrial ventilation testing may involve measuring air velocity, airflow quantity, Air Changes per Hour (ACH), airflow direction, temperature, relative humidity and pressure relationships, depending on the purpose of the assessment.
This guide explains how these measurements work, how they differ, how a ventilation survey is conducted, and how EHS professionals should interpret the results.
What Is Ventilation and Airflow Testing?
Ventilation testing is the assessment of how air enters, moves through and leaves a workplace or enclosed space.
It can be used to evaluate:
natural ventilation;
mechanical ventilation;
fresh-air supply;
general exhaust ventilation;
supply and exhaust systems;
airflow at workplaces;
airflow through openings or ducts; and
overall air-exchange performance.
The objective is not simply to obtain an anemometer reading.
A useful ventilation assessment tries to understand the actual movement and distribution of air through the workplace.
ACGIH’s current industrial-ventilation resources similarly cover make-up and supply air, dilution ventilation, exhaust systems, troubleshooting, system testing and hands-on airflow measurements as core elements of industrial ventilation practice.
What Is Measured During Industrial Ventilation Testing?
The exact parameters depend on the objective of the study.
Typical measurements may include:
Parameter
What It Tells Us
Air velocity
How fast air is moving at the measurement point
Airflow / air quantity
Volume of air moving through a duct, opening or selected area
Air Changes per Hour (ACH)
Rate at which the equivalent room air volume is replaced
Airflow direction
Direction in which air moves between areas
Temperature
Thermal conditions at the workplace
Relative humidity
Moisture conditions affecting occupants or processes
Pressure differential
Pressure relationship between selected rooms or areas
CO₂
May support occupied-space ventilation assessment where included in scope
One of the most common mistakes when reading a ventilation report is treating air velocity, airflow and ACH as if they are the same measurement.
They are not.
Air Velocity vs Airflow vs ACH
Understanding these three concepts is essential for interpreting a ventilation survey.
1. Air Velocity
Air velocity describes:
how fast the air is moving.
It may be expressed in:
metres per second – m/s
metres per minute – m/min
feet per minute – ft/min
Air velocity is commonly measured using an anemometer or other suitable air-velocity measuring instrument.
For example:
Air velocity at a workplace = 0.6 m/s
That tells us how fast the air is moving at that particular measurement location.
It does not by itself tell us the total quantity of air moving through the room.
2. Airflow or Air Quantity
Airflow describes:
how much air is moving through a known area over time.
Common units include:
cubic metres per second – m³/s
cubic metres per hour – CMH or m³/h
cubic feet per minute – CFM
In simplified form:
Airflow = Average Air Velocity × Cross-Sectional Area
Consider an opening measuring:
2 m × 1 m
Cross-sectional area:
2 m²
If representative average air velocity through the opening is:
1.5 m/s
then:
Airflow = 1.5 × 2
= 3 m³/s
or:
10,800 m³/hour
The actual measurement method needs to consider airflow distribution across the measurement plane; one point reading should not automatically be assumed to represent the entire opening.
3. Air Changes per Hour – ACH
Air Changes per Hour indicates how many times the equivalent volume of air contained within a room is replaced in one hour.
The simplified relationship is:
ACH = Airflow per Hour ÷ Room Volume
Suppose a factory room measures:
20 m × 10 m × 4 m
Room volume:
20 × 10 × 4 = 800 m³
If representative airflow is:
4,800 m³/hour
then:
ACH = 4,800 ÷ 800
= 6 ACH
This means the equivalent of the room’s air volume is being exchanged approximately six times per hour.
But there is an important point:
6 ACH is a measurement result. It is not automatically a pass or fail.
The required ventilation rate depends on factors such as:
process;
contaminants;
heat generation;
occupancy;
room use;
ventilation design;
applicable technical standards; and
applicable statutory requirements.
There is no single ACH number that should be blindly applied to every factory.
Why One Air Velocity Reading Can Be Misleading
Imagine this situation.
A large production area has an exhaust fan mounted on one wall.
An air-velocity measurement is taken directly in front of the fan.
The reading appears good.
Does this prove the whole production area has adequate ventilation?
No.
Air movement might be strong near the fan while workers 20 metres away experience almost no meaningful airflow.
Similarly, air may enter through an opening and travel directly toward an exhaust point without properly passing through the occupied work zone.
This is sometimes referred to as poor air distribution or, depending on the configuration, short-circuiting of airflow.
A ventilation assessment should therefore consider:
measurement + location + layout + airflow direction + site observations
rather than relying on one isolated number.
How Is Air Velocity Measured in a Factory?
Air velocity is generally measured using an appropriate air-velocity instrument such as an anemometer.
Depending on the purpose of the survey, measurements may be taken at:
worker locations;
fresh-air openings;
supply-air outlets;
exhaust points;
duct openings;
doors;
windows;
ventilation grilles; or
other representative positions.
The monitoring location matters.
Taking a measurement only where the airflow is strongest can produce a misleading picture of overall ventilation performance.
Representative readings should instead be selected based on the actual objective of the study.
Where Should Airflow Measurements Be Taken?
There is no universal measurement grid suitable for every factory.
The selection of monitoring points depends on:
workplace dimensions;
worker locations;
source of contaminants;
heat-producing processes;
fresh-air locations;
exhaust locations;
walls and partitions;
machinery;
storage;
ventilation configuration; and
purpose of testing.
For example, if workers are complaining about stagnant conditions around one production line, readings taken only near the fresh-air inlet may not answer the real question.
The occupied work area itself needs to be assessed.
How Is Airflow Through a Duct or Opening Calculated?
Where appropriate, air quantity can be calculated by combining representative velocity measurements with the effective cross-sectional area.
Simplified:
Q = V × A
where:
Q = airflow
V = average air velocity
A = cross-sectional area
This is why measurements across a duct or opening may require multiple velocity readings.
Velocity may not be uniform across the complete section.
A reading at the centre of an opening, for example, should not automatically be assumed to represent every point across that opening.
Supply Airflow vs Exhaust Airflow
Good industrial ventilation is not simply about supplying more air.
There must also be consideration of where the air comes from and where it goes.
Supply Airflow
Supply ventilation introduces air into the workplace.
The assessment may examine whether:
adequate air is entering;
it reaches the required area;
its distribution is appropriate; and
obstructions interfere with its movement.
Exhaust Airflow
Exhaust ventilation removes air from the workplace.
The assessment may examine:
whether the exhaust is actually drawing air;
whether airflow quantity is adequate for the intended purpose;
whether the exhaust location is appropriate; and
whether air is being removed from the area where it is needed.
ACGIH industrial-ventilation guidance covers supply/make-up air and exhaust systems as distinct but interconnected parts of ventilation-system performance.
General Ventilation vs Local Exhaust Ventilation
These concepts should also not be confused.
General Ventilation
General or dilution ventilation introduces and removes air across a larger workplace.
It may help:
control heat;
dilute low concentrations of contaminants;
reduce stale air; and
improve general workplace conditions.
Local Exhaust Ventilation
Local exhaust ventilation, often called LEV, is designed to capture a contaminant close to where it is generated.
Examples may include:
welding-fume extraction;
process hoods;
dust extraction;
laboratory hoods; and
local exhaust arms.
Where hazardous contaminants are generated, simply increasing general airflow is not necessarily a substitute for properly designed source-capture ventilation.
The appropriate control should be determined from the process and exposure risk.
Ventilation Testing Procedure in a Factory
The exact ventilation survey procedure varies with the purpose of the assessment, but a practical industrial ventilation survey typically follows several stages.
Step 1 – Understand Why Testing Is Required
Testing may be initiated because of:
poor-airflow complaints;
excessive heat;
stagnant working conditions;
audit observations;
suspected inadequate fresh air;
poor exhaust performance;
process modifications;
changes in production layout; or
a regulatory or internal requirement.
Understanding the question determines what should actually be measured.
Step 2 – Review the Workplace
Before measurement, the survey team should understand the physical configuration.
Measurement points should reflect the actual purpose of the survey.
Locations may include:
occupied working areas;
supply-air points;
exhaust points;
low-airflow locations;
openings;
ducts; and
locations identified through worker complaints or site inspection.
Step 4 – Measure Air Velocity
Air velocity is measured using an appropriate instrument.
Multiple readings may be required where airflow varies across an area.
Step 5 – Calculate Airflow Where Required
Where cross-sectional area and representative velocity can be established, airflow quantity can be determined.
The result may be expressed in:
CFM, CMH or m³/s
depending on the reporting requirement.
Step 6 – Calculate ACH Where Applicable
If room volume and representative airflow quantity are available, Air Changes per Hour can be calculated.
ACH can be useful for understanding overall air-exchange performance, but it should always be interpreted in context.
Step 7 – Observe Airflow Distribution
Numerical measurements should be interpreted together with what is happening physically.
Questions include:
Is air reaching workers?
Are there stagnant zones?
Are storage racks blocking movement?
Is fresh air immediately leaving through a nearby exhaust?
Has new machinery changed airflow?
Are doors significantly affecting the pattern?
Step 8 – Document Results
The final report should explain:
where measurements were taken;
how measurements were taken;
what equipment was used;
what the measured values were;
calculations performed;
observations made;
reference criteria where applicable; and
recommendations or further actions.
What Do We Commonly Find During Factory Ventilation Assessments?
Perfect Pollucon LLP has been providing environmental monitoring and consultancy services since 2007, supported by senior environmental professionals with up to 40+ years of field experience.
One practical lesson from ventilation work is that poor ventilation is not always solved by simply installing a bigger fan.
Ventilation problems can arise from the way air is distributed.
Conditions worth investigating include:
fresh air entering but failing to reach workers;
supply and exhaust points positioned poorly;
storage racks obstructing airflow;
modifications to machinery layouts;
exhaust systems drawing less air than expected;
blocked filters or restricted ducts;
open doors altering the intended airflow pattern;
low-airflow pockets;
unequal distribution across large work areas; and
changes to the building after the original ventilation design.
This is why field observations are an important part of ventilation testing.
What is present in the air occupants are breathing?
Depending on the monitoring programme, IAQ parameters may include:
carbon dioxide;
particulate matter;
temperature;
relative humidity; and
other parameters relevant to the facility.
A building can have measurable airflow while still having an indoor air-quality problem.
Similarly, an IAQ reading does not necessarily explain why ventilation is performing poorly.
Where required, the two assessments can complement each other.
Is There a Minimum Air Velocity Requirement for Factories in India?
This question requires care.
The Factories Act, 1948 historically addressed ventilation and temperature through Section 13, which requires adequate ventilation by circulation of fresh air and conditions providing reasonable comfort and protection from injury to health.
Model factory rules prescribed under Section 13 have also included provisions concerning temperature and air movement, including a reference value of 30 metres per minute in the circumstances covered by those rules.
However:
A single number should not be copied from a general provision and treated as the universal ventilation requirement for every factory, process or room.
Actual applicability depends on the governing state rules, process, workplace conditions and other applicable occupational-safety requirements.
Requirements for specific processes such as contaminant extraction, laboratories, cleanrooms, pharmaceutical areas or hazardous operations may also be governed by more specific technical criteria.
Therefore, ventilation results should be compared with the specific requirement applicable to the facility, not with a random ACH or velocity value found online.
Is 30 Metres per Minute the Same as 0.5 m/s?
Yes.
Unit conversion:
30 metres/minute ÷ 60
= 0.5 metres/second
Therefore:
30 m/min = 0.5 m/s
But again, the presence of this value in a particular regulatory reference does not mean 0.5 m/s becomes a universal pass/fail criterion for every ventilation measurement taken inside a factory.
Measurement location and regulatory context matter.
Is There a Standard ACH for Every Factory?
No.
This is one of the most common misconceptions around ventilation testing.
A statement such as:
“Every factory requires 6 ACH”
is generally too simplistic.
The appropriate ventilation rate can depend on:
occupancy;
type of process;
heat generation;
contaminant generation;
room use;
exhaust requirements;
ventilation design;
industry-specific requirements; and
applicable regulatory or technical criteria.
ACH should therefore be treated as a measured performance indicator, not as an automatic universal compliance number.
Can High ACH Still Mean Poor Ventilation?
Potentially, yes.
Imagine fresh air entering near the ceiling and being exhausted nearby without reaching the workers below.
The overall airflow quantity might generate an apparently reasonable ACH.
But the actual occupied zone may still be poorly ventilated.
This demonstrates why:
ACH measures air-exchange quantity, not necessarily effective air distribution.
Ventilation assessment should consider both.
Common Ventilation Problems Found in Industrial Facilities
1. Stagnant-Air Zones
Certain parts of a large production area may receive very little air movement.
2. Supply and Exhaust Short-Circuiting
Fresh air may travel directly toward an exhaust without properly ventilating the workplace.
3. Obstructed Airflow
Machinery, partitions, racks or stored material may block intended airflow paths.
4. Inadequate Exhaust
A fan may be running but removing insufficient air.
5. Poorly Positioned Exhaust
The exhaust point may be too far from the contaminant or heat source.
6. Inadequate Fresh Air
Mechanical exhaust without sufficient replacement air can affect system performance.
7. Changed Factory Layout
Ventilation may have been designed before new equipment, partitions or storage systems were installed.
8. Poor Maintenance
Filters, ducts, fan components or ventilation openings can become restricted.
When Should a Factory Consider Ventilation Testing?
Ventilation assessment may be useful when:
workers repeatedly complain of poor air movement;
parts of the workplace feel stagnant;
excessive heat accumulates;
fumes or odours remain in the working area;
new equipment has been installed;
production layout has changed;
partitions have been added;
exhaust systems have been modified;
an audit requests objective ventilation data;
a fresh-air system has been installed or changed; or
management wants to verify ventilation performance.
Periodic testing can also help identify deterioration in existing systems.
ACGIH’s current guidance specifically describes testing of ventilation systems both at initial start-up and during periodic performance checks.
What Should a Ventilation Testing Report Contain?
A useful ventilation report should enable another competent person to understand what was measured, where it was measured and what the results mean.
Depending on the scope, the report may contain:
client/facility details;
area assessed;
monitoring date;
purpose of survey;
measurement locations;
instruments used;
methodology;
air-velocity readings;
airflow calculations;
ACH calculations;
temperature/humidity measurements;
pressure measurements where applicable;
site observations;
applicable reference criteria;
photographs or location diagrams;
conclusions; and
recommendations.
A table simply containing five anemometer readings without explaining their locations or context has limited value.
How Should You Read a Ventilation Survey Report?
When reviewing a report, ask:
1. Where was the reading taken?
A reading immediately beside a fan cannot automatically represent the complete factory floor.
2. Was air velocity or airflow measured?
They are different parameters.
3. If airflow was calculated, how was area determined?
Airflow calculations depend on both velocity and cross-sectional area.
4. If ACH was calculated, what room volume was used?
Incorrect room dimensions produce incorrect ACH.
5. Were multiple representative locations assessed?
Large workplaces usually cannot be characterised properly using one reading.
6. What standard or regulatory requirement was used?
The report should not declare “pass” simply because the result exceeds an arbitrary internet value.
7. Were site observations recorded?
Airflow distribution problems often become clear only when measurements and physical observations are considered together.
A Simple Ventilation Testing Example
Consider a production room:
Length: 25 m Width: 12 m Height: 4 m
Room volume:
25 × 12 × 4
= 1,200 m³
Suppose representative airflow through the ventilation system is determined to be:
7,200 m³/hour
Then:
ACH = 7,200 ÷ 1,200
= 6 ACH
The correct interpretation is:
The calculated air-change rate is approximately 6 air changes per hour under the conditions measured.
The incorrect interpretation would be:
The factory has 6 ACH, therefore ventilation automatically complies.
Whether six air changes are adequate depends on what the room is used for and what requirement applies.
That distinction is fundamental to professional ventilation assessment.
Which Instruments Are Used for Ventilation Testing?
Instrument selection depends on the objective.
Equipment may include:
Anemometer / Air Velocity Meter
Used to measure air velocity.
Temperature and Humidity Instrument
Used where thermal or moisture conditions form part of the assessment.
Differential Pressure Instrument
May be used where pressure relationships between spaces are relevant.
Smoke Tube or Airflow Visualisation Method
May help understand airflow direction or identify unexpected air movement where appropriate.
Indoor Air Quality Instruments
May be included where parameters such as CO₂ or particulate matter form part of the agreed scope.
Instruments should be appropriate for the measurement range and maintained/calibrated in accordance with the organisation’s monitoring procedures.
Industrial Ventilation Testing by Perfect Pollucon LLP
Perfect Pollucon LLP conducts industrial ventilation surveys and airflow testing for factories and commercial workplaces.
Perfect Pollucon has been providing environmental monitoring and consultancy services since 2007, with senior environmental professionals bringing up to 40+ years of field experience.
Depending on the facility and monitoring objective, the scope may include:
air-velocity measurement;
airflow assessment;
Air Changes per Hour calculations;
supply-air assessment;
exhaust-airflow assessment;
airflow-direction observations;
temperature and relative humidity;
pressure differential where applicable; and
related indoor air-quality measurements where included in scope.
The findings are documented in a technical report containing relevant measurement locations, results, calculations, observations and recommendations.
Planning a Ventilation Survey?
To help determine the appropriate monitoring scope, share:
factory/facility location;
type of industry or process;
approximate area to be assessed;
reason for ventilation testing;
specific worker complaint, if any;
whether mechanical ventilation/exhaust is installed; and
any audit, client or regulatory reference requirement.
This helps the survey team understand the actual question before deciding what measurements are required.
For industrial ventilation surveys and airflow testing, contact Perfect Pollucon LLP.
Frequently Asked Questions
What is airflow testing?
Airflow testing measures the movement of air through a workplace, duct, opening or ventilation system. Depending on the purpose, testing may involve air velocity, airflow quantity, airflow direction and air-change calculations.
What is an industrial ventilation survey?
An industrial ventilation survey evaluates air movement and ventilation performance within a factory or workplace using measurements together with site observations.
How is air velocity measured?
Air velocity is typically measured using an appropriate anemometer or air-velocity meter at selected representative locations.
What is the difference between air velocity and CFM?
Air velocity measures how fast air is moving. CFM measures the volume of air flowing per minute. Airflow quantity depends on both velocity and the area through which the air is moving.
What does ACH mean?
ACH means Air Changes per Hour and indicates how many times the equivalent room air volume is replaced within one hour.
How do you calculate ACH?
A simplified calculation is:
ACH = Hourly Airflow ÷ Room Volume
The units must be consistent.
Is higher ACH always better?
Not necessarily. Required ventilation depends on the facility and process, and effective air distribution is also important. High airflow that bypasses occupied areas may not solve a ventilation problem.
Is there one minimum ACH for every factory?
No. The appropriate ventilation rate depends on the workplace, process, contaminants, heat, occupancy, ventilation design and applicable regulatory or technical requirements.
Can airflow testing identify stagnant areas?
Representative measurements combined with observations can help identify areas with relatively low or poor air movement.
Is ventilation testing the same as indoor air quality testing?
No. Ventilation testing primarily evaluates air movement and exchange. IAQ testing evaluates the quality of the air, although the two assessments can be combined where required.
What is supply airflow?
Supply airflow is air introduced into a workplace through natural or mechanical ventilation.
What is exhaust airflow?
Exhaust airflow is air removed from the workplace by an exhaust or extraction system.
What should I provide when requesting ventilation testing?
Provide the facility location, industry/process, approximate testing area, reason for assessment and any specific audit or technical requirement. This helps define the correct monitoring scope.
Technical References
Industrial ventilation is a specialised engineering and occupational-health subject. Applicable requirements should be determined from the specific facility, process and governing rules.
Useful technical references include:
applicable Indian factory and occupational-safety requirements;
applicable state factory/occupational-safety rules;
ACGIH industrial ventilation guidance;
industry-specific ventilation standards; and
applicable client, process or facility-design requirements.
ACGIH currently publishes dedicated guidance for industrial ventilation design, operation, maintenance and system testing, including its Industrial Ventilation manuals and Guide for Testing Ventilation Systems.
Technical Review
This article should be technically reviewed by Perfect Pollucon LLP’s senior environmental professionals before publication.
Tanaji S. Gajare Founder & Chairman 40+ years of professional experience in environmental monitoring and pollution-control work
Kunal Gajare Environmental Engineer 10+ years of professional experience in environmental monitoring and compliance work
Last technical review: September 2026
Resources:
Section 13 of the Factories Act, 1948 addresses ventilation and temperature in factories.
ACGIH publishes dedicated technical guidance on industrial ventilation, including supply air,
exhaust systems, troubleshooting and ventilation-system testing. See ACGIH Industrial Ventilation .
