What is TR19 Air?
TR19® Air is the Building Engineering Services Association (BESA) guidance on the internal cleanliness of ventilation systems. The current edition is Version 2, published in March 2026. It sets out how systems should be inspected, how cleanliness is measured, when cleaning is justified and how results and cleaning works should be reported.
Kitchen extract systems are covered by separate guidance, TR19® Grease, because grease deposits create a different fire risk and are measured differently.
Is TR19 a legal requirement?
No. TR19 is industry guidance, not legislation. But it is the recognised way of showing that ventilation systems are being managed properly, and it is often required indirectly:
- the Regulatory Reform (Fire Safety) Order 2005 requires fire risks to be assessed and managed, and ductwork deposits can form part of that risk
- the Workplace (Health, Safety and Welfare) Regulations 1992 require effective and suitable ventilation
- insurers, landlords and FM contracts frequently require TR19 evidence
What applies to your building depends on the system type, building use, contractual arrangements and risk assessment.
Why ventilation hygiene matters
- Deposits in ductwork can add to fire load and help fire spread
- Dirty systems can release dust into occupied spaces and contribute to complaints
- Contaminated coils, filters and ductwork reduce system performance and raise energy use
- Without evidence, it is difficult to show insurers, landlords or auditors that the risk is being managed
How cleanliness is measured
A TR19 assessment combines visual inspection with a quantitative test. No single test suits every system, so the method should be chosen to suit the duct surface, deposit and purpose.
| Method | What it measures | Typically used for |
|---|---|---|
| Visual inspection and photography | Condition, visible deposits and damage | Every assessment |
| Deposit Thickness Test (DTT) | Thickness of dry deposit, in microns, on site | Suitable flat metal duct surfaces |
| Vacuum test (EVT, formerly PVT) | Mass of dust per square metre, by weight | Quantitative verification, and surfaces where DTT is unsuitable |
| Wet Film Thickness Test (WFTT) | Depth of wet or soft grease, in microns | Kitchen extract systems |
How often should systems be inspected?
TR19 Air sets inspection intervals by system type, building use and contamination risk rather than one fixed schedule. Most commercial supply and extract systems are inspected at least annually, with shorter intervals for higher-risk systems. For kitchen extract, TR19 Grease links frequency to hours of use: as a guide, every 3 months for heavy use, 6 months for moderate use and 12 months for light use.
What should happen after an assessment?
A good assessment ends with one of three outcomes: no action and a next inspection date; targeted cleaning or access improvements with a defined scope; or further investigation where the findings point to a wider indoor air quality or ventilation problem.
If cleaning is carried out, it should be verified afterwards against the pre-clean results. Version 2 of TR19 Air places more emphasis on post-clean reporting, including before-and-after photographs, test results and a record of inaccessible areas.
Why use an independent assessor?
Companies that both inspect and clean ductwork have an incentive to recommend cleaning. Green Air Monitoring does not provide cleaning services. We assess, specify and verify, so our recommendations are based only on the evidence.
Find out more about our TR19 Air ventilation hygiene assessments, duct cleanliness testing and ventilation hygiene PPM contracts for FM portfolios across London and the South East.