HSE – Measuring Clearance Times in Spray Booths

This HSE guidance provides a practical method for measuring the clearance time of spray booths and spray rooms using a smoke or fog generator. Clearance time is the period required for the ventilation system to remove airborne contaminants after spraying has stopped and before workers can safely enter the enclosure without respiratory protection.

The guidance was developed primarily for the motor vehicle repair industry where isocyanate-containing paint products present significant occupational health risks. However, the principles are equally applicable to many enclosed processes involving hazardous dusts, mists, fumes or vapours. Understanding and displaying clearance times is an important part of managing exposure risks and ensuring that workers remain protected until airborne contaminants have been effectively removed.

What This Resource Covers

This guidance covers:

What spray booth clearance time means
Why clearance times are important
Smoke testing procedures
Measuring ventilation effectiveness
Identifying stagnant zones and airflow dead spots
Displaying clearance times for operators
Protection from isocyanate paint mist exposure
The relationship between clearance testing and LEV Thorough Examination and Test

Source Document

View the HSE guidance here:

View Original HSE Guidance

Source: Health and Safety Executive (HSE)
Document Type: COSHH Guidance
Status: Current 2026
Last reviewed by LEVCentral: June 2026

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Why This Guidance Matters

One of the greatest risks associated with spray painting operations is the mistaken belief that airborne contaminants disappear as soon as spraying stops.

In reality, fine paint mist can remain suspended within a spray booth or spray room long after application has finished. HSE notes that spray spaces should have their clearance time established so workers know how long they must wait before removing respiratory protective equipment or entering the enclosure without protection.

This is particularly important when spraying isocyanate-containing paints, which remain one of the leading causes of occupational asthma in the UK. Exposure may occur even after spraying has ceased if workers enter the enclosure too early or remove respiratory protection before contaminants have been fully cleared.

The HSE Smoke Testing Method

The HSE procedure uses a smoke or fog generator to completely fill the spray booth or room before the extraction system is activated.

The process involves:

Turning off the extraction system
Filling the enclosure with smoke
Activating the ventilation system
Measuring the time taken for all visible smoke to disappear
Checking for air leaks and dead spots during the test
Rounding the measured time up to the next quarter minute
Displaying the clearance time at the booth entrance

The guidance recommends undertaking the test just before filter replacement to establish a worst-case clearance time.

Clearance Times and Worker Protection

Once established, the clearance time should be clearly displayed and communicated to all personnel using the booth or room.

The clearance period helps determine:

When respiratory protective equipment can be removed
When workers can safely enter the enclosure
Whether ventilation performance has deteriorated
Whether maintenance or filter replacement may be required

HSE guidance highlights that workers should understand and observe these times consistently as part of normal operating procedures.

Key LEVCentral Takeaways

Clearance time measures how long contaminants remain airborne after spraying.
Smoke testing provides a practical method for measuring clearance time.
Clearance times should be established under worst-case conditions.
Results should be displayed at the spray booth or spray room entrance.
Workers should not remove RPE until the clearance period has elapsed.
Smoke testing can identify leaks, stagnant zones and airflow problems.
Clearance time testing can form part of the LEV Thorough Examination and Test process.
Understanding clearance times is particularly important where isocyanate paints are used.

Further Resources

Related LEVCentral Resources:

Recommended Learning

Thought Leadership

Smoke clearance testing is a simple but powerful reminder that contaminants often remain present long after the visible work activity has ended. By establishing and displaying realistic clearance times, employers move beyond assumptions and provide workers with clear, evidence-based instructions on when it is safe to enter a spray space or remove respiratory protection.

From an occupational hygiene perspective, clearance time testing transforms an invisible risk into a measurable and manageable control parameter. It is a practical example of how visualisation techniques can be used to improve both compliance and worker protection.