Heat treatment (HT) is the most widely used ISPM 15 treatment. It heats wood packaging in a chamber until the coldest point in the wood, the core, reaches at least 56 °C (132.8 °F) and holds it there for at least 30 minutes without interruption. At that temperature and time, insects, nematodes and many other pests in the wood are killed.
The requirement in detail
The standard defines HT by the temperature of the wood, not by the temperature of the air in the chamber. Three elements must all be met:
- Minimum core temperature: 56 °C throughout the entire profile of the wood, including the core of the thickest and slowest-heating pieces.
- Minimum time: 30 minutes continuously at or above that temperature. The 30 minutes start only when the coldest point reaches 56 °C, not when the kiln door closes.
- Continuity: a drop below 56 °C interrupts the clock, so the period must be uninterrupted.
The chamber therefore has to run far hotter than 56 °C, or run for hours, because wood is a poor heat conductor. The overall cycle length depends on species, thickness, starting moisture content and load density. Wet or thick timber takes much longer than dry, thin boards. Exact schedules are set by each operator and validated for each kiln; they are not fixed by the standard.
The same standard recognizes other measures: methyl bromide (MB), dielectric heating (DH) and sulfuryl fluoride (SF). HT carries the code HT in the stamp (the ISPM 15 mark).
| Parameter | HT | DH (for comparison) |
|---|---|---|
| Energy | Hot air or steam in a chamber | Microwave or radiofrequency |
| Target temperature | 56 °C at the core | 60 °C throughout the wood profile |
| Hold time | 30 minutes continuously | 1 minute |
| Typical equipment | Kilns and heat chambers | Dielectric heating units |
Chambers and kilns used for HT
Several types of equipment are used, from purpose-built heat treatment kilns to adapted lumber drying kilns:
- Conventional kilns heated by steam or hot water coils, often fired by wood waste, gas or oil. These are the commonest design for pallet plants.
- Direct-fired or hot-air chambers, which blow heated air across the stack.
- Dehumidification kilns with heat pumps, which can be adapted for HT if they reach and hold the required core temperature.
- Container-based or mobile units, used by smaller producers or for treating wood on site.
Chamber layout should circulate air evenly so that no part of the load lags behind. Stacking pallets with gaps between layers lets hot air reach every piece; a dense, poorly stacked load creates cold spots. Equipment types are compared in heat treatment kiln equipment, and operating practice, including loading and cycles, is covered in heat treatment kiln operation.
Monitoring: probes and records
Compliance is shown by data. Temperature probes are inserted into the wood, in the thickest pieces and in the slowest-heating position in the chamber, and connected to a data logger that records temperature over time. The log proves that the core reached 56 °C and for how long. Practical requirements include:
- Calibrated sensors, checked regularly against a reference.
- Placement at the coldest points, found by testing the chamber and repeating it when loads or stacking patterns change.
- Tamper-resistant records, with batch number, date, load description and the temperature curve.
- Authorization: in most countries the facility and its procedures are approved by the NPPO, which audits records and the stamp use. Türkiye, for example, requires operators to keep computerized temperature records and treatment documents for two years under its national regulation (see pallets in Türkiye).
The certification and audit steps are explained in ISPM 15 certification, and the paper trail in ISPM 15 documentation.
Kiln drying is not automatically heat treatment
Pallet producers dry lumber in kilns to reduce moisture, and the temperatures can be high. But ISPM 15 treats kiln drying (KD) as HT only when the kiln schedule and monitoring demonstrate that the wood reached the HT parameters. A wood that has been kiln-dried to 20% moisture content, for example, has not been heat treated if its core never reached 56 °C for 30 minutes under monitoring, and a "KD" mark is not a valid ISPM 15 code. The distinction is explained in kiln-dried vs heat-treated. Moisture itself is a separate quality factor; see wood moisture content.
What HT does and does not do
- It kills pests present in the wood at the time of treatment, including nematodes and wood-boring insects.
- It does not give lasting protection. Treated wood can be reinfested or can grow mold if it is stored wet or in poor conditions; see mould and blue stain.
- It does not substitute for debarking, which the standard requires separately; see the debarking requirement.
- It leaves wood chemically unchanged, unlike fumigation with methyl bromide or sulfuryl fluoride, so there are no fumigant residues to manage.
Because it needs no chemicals and is widely available, HT is the standard choice for pallets, and for exporters it is the safest treatment to specify. The big picture is in ISPM 15 explained.
Frequently asked questions
What temperature does heat treatment require under ISPM 15?
The core of the wood must reach at least 56 °C (132.8 °F) and stay there for at least 30 continuous minutes.
Is the chamber temperature or the wood temperature measured?
The wood temperature at its core. Air temperature in the chamber is higher and is not the compliance measure.
How long does an HT cycle take?
It varies with species, thickness, moisture and load density, and can range from a few hours to much longer. The operator validates a schedule for each kiln.
Is kiln-dried wood the same as heat-treated wood?
Only if the kiln process and records show the HT parameters were met. A KD stamp is not an ISPM 15 mark.
Sources
- ISPM 15 (2018 edition, Korean translation) — International Plant Protection Convention
- ISPM 15 — Wikipedia
- ISPM 15 Annex 1 background — International Plant Protection Convention
- ISPM 15: inception to current implementation — Standards and Trade Development Facility
- Ahşap Ambalaj Malzemelerinin Isıl İşleme Tabi Tutulması ve İşaretlenmesine Dair Yönetmelik (guidance) — Ministry of Agriculture and Forestry, Türkiye