Dielectric heating (DH) is an ISPM 15 treatment that uses microwave or radiofrequency energy to heat wood packaging from the inside. Under the standard, the wood must reach at least 60 °C (140 °F) throughout its entire profile, including the surface, and hold it for a minimum of one continuous minute. DH was approved in 2013 and is marked with the code DH.
How dielectric heating works
Wood contains water, and water molecules are polar. In an alternating electromagnetic field the molecules continually reorient, producing heat within the wood itself. Microwave systems use very high frequencies; radiofrequency systems use lower ones. Unlike a conventional kiln, which heats the surface with hot air and waits for heat to conduct inward, DH heats the whole cross section more or less at once. This is why the process takes minutes instead of hours.
A DH installation typically has a shielded treatment tunnel or chamber, a conveyor or a loading system, a generator that produces the field, and temperature monitoring equipment. Pallets or components pass through, or are held in, the chamber for a controlled time.
The ISPM 15 requirements
According to the ISPM 15 text and national implementations:
| Requirement | Value |
|---|---|
| Minimum temperature | 60 °C throughout the entire profile of the wood, including the surface |
| Minimum duration | 1 minute, continuously |
| Time to reach temperature | within 30 minutes from the start of treatment |
| Maximum wood dimension | smallest dimension of the cross-section no larger than 20 cm |
| Additional conditions | wood thicker than 5 cm needs particular processing conditions in the standard's schedule |
| Stamp code | DH |
Compare these with heat treatment, where the core must reach 56 °C and stay there for 30 minutes (heat treatment). DH requires a higher temperature but only for a minute, and the standard requires the temperature at the surface as well as in the core. That matters, because in a microwave field the interior is often hotter than the outside, and a cold surface can fail the requirement.
The standard history is in ISPM 15 explained: the standard was first adopted in 2002 and DH was added in a revision approved at the eighth session of the Commission on Phytosanitary Measures in 2013, with further revisions to the dielectric heating text considered in 2015–2017 consultations.
DH compared with HT, MB and SF
| HT | DH | MB | SF | |
|---|---|---|---|---|
| Principle | Hot air or steam | Microwave or radiofrequency | Gas fumigation | Gas fumigation |
| Treatment time | Hours (cycle) | Minutes | 24 hours | 24–48 hours |
| Chemical residues | None | None | Possible fumigant | Possible fumigant |
| Capital cost | Moderate | High | Low to moderate | Low to moderate |
| Typical user | Most pallet plants | Large or automated plants | Declining | New alternative |
Treatment times and costs are qualitative comparisons; actual figures depend on the equipment and wood. See methyl bromide and sulfuryl fluoride for the chemical options.
Advantages
- Speed: minutes per load, so treatment can be built into a continuous production line and wood does not wait for a kiln cycle.
- No chemicals: like heat treatment, DH leaves no fumigant residue.
- Fits automation: conveyor-fed units can be integrated into an automated plant; see automation in pallet production.
Limits and cautions
- Equipment cost and know-how: DH units cost considerably more than a basic kiln, and operators need skills in electromagnetic systems and safety (shielding against radiation).
- Thickness limit: the standard schedule covers wood with a smallest cross-section dimension of up to 20 cm, which includes nearly all pallet components.
- Non-uniform heating: moisture content, density and the presence of metal such as nails influence how energy is absorbed, which can create hot or cold spots. Temperature must be proven at the coolest location, not just the average.
- Moisture: very wet wood may heat unevenly and release steam; see wood moisture content.
- Acceptance: because DH is a newer treatment, check that the importing country accepts DH-marked packaging before shipping; most NPPOs follow the standard, but individual rules vary. See ISPM 15 country rules.
Who uses DH?
DH is available in countries that have approved it in their national rules. Türkiye's regulation, for instance, names both heat treatment and dielectric heating as the permitted methods for marking wood packaging, with DH described for material under 20 cm in thickness (see pallets in Türkiye). Reliable global statistics on how much packaging is treated by DH are not available, and in most markets the share is small compared with conventional HT.
Facilities using DH go through the same authorization and audit steps as other producers: a registered facility, validated schedules, calibrated temperature sensing, records and controlled use of the mark. See ISPM 15 certification.
Frequently asked questions
What does DH mean on a pallet stamp?
DH shows that the wood was treated by dielectric heating (microwave or radiofrequency) to at least 60 °C throughout its profile for at least one minute.
When was DH added to ISPM 15?
It was approved by the Commission on Phytosanitary Measures in 2013, eleven years after the standard was first adopted.
Is DH better than heat treatment?
It is much faster and suited to automated lines, but the equipment is more expensive and heating must be proved at the coolest part of the wood. Most producers still use conventional HT.
Is there a size limit for DH?
Yes. The standard schedule applies to wood up to 20 cm in its smallest cross-section dimension.
Sources
- ISPM 15 (2018 edition, Korean translation) — International Plant Protection Convention
- ISPM 15: inception to current implementation — Standards and Trade Development Facility
- ISPM 15 Annex 1 background — International Plant Protection Convention
- Ahşap Ambalaj Malzemelerinin Isıl İşleme Tabi Tutulması ve İşaretlenmesine Dair Yönetmelik (guidance) — Ministry of Agriculture and Forestry, Türkiye
- ISPM 15 — Wikipedia