HSE – Measuring Paint Spray Booth Clearance Times

Determining Safe Re-entry Times Using Smoke Testing

 

LEVCentral Expert Commentary

One of the most important safety features of a spray booth is often one of the least understood — Clearance Time.

When spraying has stopped, hazardous airborne paint mist does not disappear immediately. Fine aerosol particles, particularly those containing isocyanates, can remain suspended within the booth long after the visible cloud has disappeared. Workers who remove their air-fed respiratory protective equipment (RPE) too soon, or enter the booth before contaminants have been removed, may still receive a significant exposure.

This HSE guidance explains a simple but highly effective procedure for measuring spray booth clearance times using a smoke or fog generator. By filling the booth with smoke before switching on the extraction system, engineers can determine how long the ventilation system takes to remove airborne contaminants under worst-case conditions. The measured clearance time should then be displayed at the booth entrance and observed by all users.

The guidance is particularly relevant to organisations spraying two-pack isocyanate paints, but the principles apply equally to many enclosed processes where airborne dusts, fumes, vapours or mists remain suspended after the process has finished.

For LEV professionals, this guidance reinforces an important principle:

An LEV system is judged not simply by the airflow it produces, but by how effectively it removes hazardous contaminants from the workplace.

View HSE Guide

Related HSE Research Report

The practical guidance is supported by an HSE research project which investigated the development of an automatic spray booth clearance time indicator.

RR742 investigated an inexpensive electronic system that automatically starts a clearance timer when spraying finishes. The report evaluated different sensing methods and concluded that automatic indication systems could significantly reduce the likelihood of operators removing RPE before the booth had fully cleared.

Key Learning Points

The guidance explains:

• What spray booth clearance time means.
• Why airborne paint mist remains after spraying has stopped.
• The HSE smoke testing procedure.
• Measuring clearance times under worst-case conditions.
• Why testing should normally be carried out immediately before filter replacement.
• Identifying air leaks and stagnant areas during smoke testing.
• Recording and displaying clearance times.
• Why workers must not remove RPE until the clearance period has elapsed.
• Incorporating clearance time testing into the LEV Thorough Examination and Test.
• The potential use of automatic clearance-time indicators.

Source Document Information

Organisation: Health and Safety Executive (HSE)

Document: Measuring Clearance Times in Spray Booths

Supporting Research: RR742 – An Automated System for Indicating Spray Clearance Times of MVR Spray Booths and Rooms

Document Type: HSE Guidance and Research Report

Primary Topics: Spray Booths, Clearance Time, Smoke Testing, Isocyanates, LEV, Thorough Examination and Test

Audience: LEV Engineers, Spray Booth Inspectors, Occupational Hygienists, Vehicle Bodyshops, Spray Booth Manufacturers, Health & Safety Professionals and Duty Holders.

LEVCentral Perspective

This guidance represents an excellent example of performance verification.

Many spray booths are commissioned using airflow measurements, face velocities and pressure readings. Whilst these measurements are important, they do not directly answer the question that matters most to operators:

“When is it safe to remove my breathing apparatus?”

Clearance time testing provides that answer.

By visualising contaminant removal with smoke, engineers can demonstrate whether the booth effectively removes airborne contaminants and determine the waiting period required before personnel can safely enter or remove RPE.

The associated HSE research (RR742) is equally interesting because it recognises that relying entirely on operators remembering clearance times introduces the possibility of human error. Automatic timing systems provide an additional layer of protection by indicating when the required clearance period has elapsed. Although relatively simple, this concept represents an excellent example of designing human factors into engineering controls rather than relying solely on procedures.

Further Resources

• HSG276 – Isocyanate Paint Spraying: Safely Managing Spray Booths and Rooms
• HSG258 – Controlling Airborne Contaminants at Work
• INDG388 – Safety in Isocyanate Paint Spraying
• HSE 2-Pack Isocyanate Paints

Recommended Learning

• M200 Basic Principles in Occupational Hygiene
• M505 Control of Hazardous Substances
• M507 Health Effects of Hazardous Substances
• P600 Methods for Testing Performance of LEV
• P601 LEV Thorough Examination & Testing
• P602 LEV Basic Design Principles
• P604 LEV Commissioning & Performance Evaluation

Thought Leadership

Clearance time testing highlights an important distinction between ventilation performance and health protection.

An extraction system may achieve its design airflow yet still leave hazardous contaminants suspended within an enclosure for a significant period after work has stopped. Measuring clearance time bridges this gap by converting engineering performance into practical operational guidance that workers can follow.

The accompanying HSE research is particularly forward-thinking because it moves beyond measurement alone and considers how engineering controls can be integrated with human behaviour. Automatic clearance-time indicators reduce reliance on memory and judgement, helping ensure that workers do not remove respiratory protection prematurely.

From a LEVCentral perspective, this perfectly illustrates the wider principle that effective LEV is not simply about moving air—it is about providing demonstrable, evidence-based protection for the people who depend upon it every day.