Heat treatment of wood: what ISPM 15, the USDA and the Wood Handbook say

This page reports what the official source says. It is not legal advice: ask the office named here. It describes three kinds of source: the International Plant Protection Convention's standard ISPM 15, two sections of the US federal import and treatment rules (7 CFR 319.40-3 and 7 CFR 305.8), and chapter 20 of the Wood Handbook from the US Forest Service's Forest Products Laboratory. State rules on moving firewood are on the moving wood and firewood page; this page covers the heat standard those rules often point to.

What ISPM 15 covers

ISPM 15 is titled Regulation of wood packaging material in international trade. Its scope is pallets, crates and dunnage made from raw wood that moves between countries. It leaves out packaging made from wood processed so that it is free of pests, and gives plywood as the example. The standard says its measures do not give ongoing protection from pests that arrive later. Its approved measures are debarked wood plus one approved treatment, with a mark that shows the treatment was done. The version read for this page was adopted in 2018 and published in 2019.

Annex 1 allows bark to remain only as small, separate pieces: under 3 cm wide whatever the length, or wider than 3 cm with each piece under 50 square cm of surface. For heat treatment the bark may come off before or after the treatment. For methyl bromide and sulphuryl fluoride it has to come off first.

The temperature and time

Treatment	Mark code	Minimum in the standard
Conventional steam or dry kiln heat chamber	HT	56 °C for 30 continuous minutes throughout the whole profile of the wood, core included.
Dielectric heating (microwave or radio frequency)	DH	60 °C for 1 continuous minute throughout the whole profile, surface included.
Methyl bromide	MB	A fumigation, not a heat treatment: concentration and time set in the standard's table, and not for packaging with a piece over 20 cm in its smallest dimension.
Sulphuryl fluoride	SF	A fumigation, listed in the mark table.

For the heat chamber, the standard says the temperature is to be checked at the coldest spot, which is the place that takes longest to warm. It says the core can be measured with sensors, or a schedule can be built from test runs that tie core temperature to chamber air temperature, allowing for moisture, species, thickness, air flow and humidity. It recommends at least two sensors, set at least 30 cm from the end of a piece. Treatment schedules are to be specified or approved, and treatment providers approved, by the national plant protection organization. Records of treatments and calibrations are kept for a period that organization sets.

The mark

Annex 2 says the mark has four parts: a symbol, an ISO two-letter country code, a code for the producer or treatment provider, and a treatment code of HT, DH, MB or SF. The mark has to be visible and legible without a visual aid, in a rectangle or square with a border.

The US import rule

Section 319.40-3(b) of 7 CFR, on the eCFR, sets conditions for importing regulated wood packaging material under a general permit. The material has to be treated under part 305 of the chapter. It has to carry a legible, permanent mark approved by the IPPC that shows a symbol, the country code, the producer number and the treatment type, with HT for heat treatment and MB for methyl bromide given as examples. An inspector at the first port can order material without the mark re-exported. The same section lets regulated articles from Canada, other than certain citrus-family items and ash (Fraxinus), come in under a general permit, with the ash sent to a separate rule in section 319.40-5(n). For the Mexican states next to the United States it lists mesquite wood for cooking, unmanufactured firewood and small packages of wood for personal cooking or medicinal use.

Section 305.8, headed heat treatment requirements, adds the facility side: the facility must be certified by APHIS, with recertification every year or as often as APHIS directs. A US facility works under a compliance agreement and one outside the United States under a workplan. The rule says the treatment period begins once the temperature in the schedule has been reached, and that an APHIS official may abort a treatment if reaching it takes an unreasonably long time.

What the Wood Handbook adds

Chapter 20 of the Wood Handbook (General Technical Report FPL-GTR-282) restates the 56 °C, 30 minute rule and the 60 °C, 1 minute dielectric rule. It says that time to heat the centre depends on the energy source, the heating medium, air circulation, the species and size of the wood, and how it is stacked, and that wet steam heats the centre fastest. It lists estimated heating times for ponderosa pine and Douglas-fir boards and timbers by thickness, wet-bulb depression and starting temperature, as starting points for schedules rather than guarantees.

The chapter says some pests need more than the packaging standard asks, and gives emerald ash borer as the example, with a minimum core temperature of 60 °C for 60 minutes, citing a 2016 USDA APHIS PPQ source. It describes a second product type, KD HT, for wood heat treated and also dried to a maximum moisture content of 19 percent or less. It says the American Lumber Standards Committee's rules call for third-party inspection of heat treatment facilities, with a verification study in certain cases, and recalibration or replacement of equipment that varies by more than 2.8 °C. It also says heat sterilization kills only the mold, fungus and insects present at the time, and that mold has quickly taken hold in heat-treated lumber that was not dry.

The drying side of the same wood is on the wood moisture and drying page.