Carboxylic acids in wood and composite wood materials

This article was triggered by the following request:
“After my new wooden house started smelling unpleasant, I had an indoor air quality assessment carried out. High levels of acetic acid were found. Now I’m worried about my family’s health. How does acetic acid affect our health? How long will we have to deal with this unpleasant smell? How can we reduce the high levels of acetic acid?”

For years, there have been discussions about terpenes, the main component of the essential oils produced in plants, including wood. These can occur in particularly high concentrations in fresh pine wood, but for production reasons also in some composite wood materials such as oriented strand board (OSB). The smell is perceived as “disturbing” and the emissions can be harmful to human health at high concentrations such as inside buildings, which also results in complaints.
In the past, there were fewer “complaints” regarding elevated concentration levels of carboxylic acids – in particular acetic and formic acid (see info box at the end of this article for more information).

Acetic acid as a natural component of wood

Wood emits acetic acid naturally, especially hardwoods, and – similar to terpenes, which occur mainly in softwoods – poses no risk to healthy people at normal concentration levels. Rather, they are partly responsible for the typical smell of wood, which many builders and homeowners explicitly want.

Natural wood emissions are often used as an argument against the use of wood, but this is completely unjustified. Only at very high concentration levels do they have a sensitizing potential, which can lead to skin and respiratory irritation or headaches. In particular, people with allergies or chemical sensitivities can experience health problems if a given room has excessive levels of such substances as acetic acid from oak or terpenes from softwood products (floor + furniture + wall/ceiling).

Elevated production-related acetic acid emissions from composite wood materials

Compared to solid wood products or wood chips, finished particleboards and flexible natural fiberboards generally release a higher amount of volatile organic acids such as acetic and formic acid. The reason for this are manufacturing processes some of which involve high temperatures. Another unwelcome by-product is the aldehyde furfural that is even more toxic (see info box at the end of this article for more information).

In recent years, there has been an increase in complaints about significantly elevated acetic acid levels in the indoor air, and not only in “older prefabricated houses.” There were also massive complaints about new buildings because of significantly elevated levels of acetic acid in connection with flexible wood fiberboard insulation and OSB panels made by some manufacturers. Particularly when the two products were used in combination, these problems even made the buildings “uninhabitable.”

In several cases, the German Environment Agency’s Indoor Air Guide Value II (RW II) of 1000 µg/m³ was significantly exceeded. The material tests also showed that the LCI value (1200 µg/m³) with concentration levels up to 4500 µg/m³ was significantly exceeded (LCI stands for “Lowest Concentration of Interest” and are set by the Committee for the Health Assessment of Building Products AgBB, German Institute for Construction Technology). According to the Standard of Building Biology Testing Methods, a value above 1000 µg/m³ is considered an “extreme anomaly.”

Nevertheless, some organizations that award quality labels still issue a certificate for such products – despite the fact that LCI values are significantly exceeded – stating that the emissions are merely “natural.”

Regarding elevated levels of acetic acid, in one case furfural values were still found in composite wood materials two years after installation, which at 70 µg/m³ even exceeded the LCI value by a factor of 7.  

Chemical building products with acetic acid

Numerous chemical building products also emit acetic acid in various concentrations – including a number of silicones used in construction. During new construction, they can represent a massive exposure, especially for sensitive people. After a few weeks, however, it is generally to be expected that the acetic acid emissions will no longer be noticeable.

It is unfortunate that “quality labels” like EC1 or EC1 Plus (issued by Emicode, a low-emission certification system for building products) do not consider acetic acid levels in their VOC assessment at all.

Acetic acid can also be emitted from other products, such as linoleum and natural paints, but at comparatively low concentration levels according to the current body of knowledge.

Acetic acid emissions: duration – remediation – liability?

In contrast to acetic acid emissions from chemicals in building products, which decrease relatively quickly, or acetic acid emissions from solid wood, which usually decrease in the medium term, acetic acid in composite wood materials can in many cases be produced again and again due to chemical reactions (the origin of acetic acid lies in the hydrolysis of acetyl groups of hemicelluloses, which make up approximately one third of the carbohydrates in wood). This is why elevated levels of acetic acid are still found in prefabricated houses that are decades old, which originate from the particleboards used at the time.

Therefore, ventilation does not necessarily bring relief. An increased supply of oxygen can even stimulate the formation of acetic acid.

“While normal ventilation leads to a significant reduction in the indoor air concentration levels of terpenes and longer-chain aldehydes, this ventilation effect is much less pronounced for formaldehyde, formic acid, and acetic acid. Ventilation can even lead to an increase in concentration for the latter.”

Dipl.-Chem. Dr. Wigbert Maraun | Zitat aus “Gebäudeschadstoffe und Innenraumluft”, Band 6

In the case of very high indoor concentration levels of acetic acid caused by composite wood materials, a truly “sustainable” remediation can therefore only be achieved by removing all materials causing the problem.

There is no way to make reliable predictions for the time course of decay as these processes are caused by reactions in the composite wood materials, depending on how the materials were manufactured, what type of adhesive was used, and what indoor climate, temperature, and exposure are prevalent.

In the event of elevated indoor exposure levels, the sources should be identified using material testing. And in the event of “exceeded” LCI values, the manufacturers should be held responsible. Finally, the MVV TB (Model Administrative Provisions – Technical Building Rules published by the DIBT, the German technical approval body) stipulates the following:

“According to sec. 3 and sec. 13 of the German Model Building Code (MBO1), building installations”must be designed, erected, modified, and maintained in such a way that public safety and order and, in particular, life, health, and the natural basis of life are not endangered. Building installations must be designed, constructed, and usable in such a way that water, moisture, plant and animal pests, and other chemical, physical, or biological agents do not result in risks or unacceptable nuisance.“

Building materials must be selected in such a way that these requirements for the building are met. It is not sufficient if they do not exceed AgBB values, for example. The Committee for Health-related Evaluation of Building Products AgBB, German Institute for Construction Technology (DIBT) (Chapter 4.2, AgBB evaluation scheme) states:

“Health-related evaluation of a building product is based on the indoor air concentrations of volatile organic compounds emitted from that product. The evaluation cannot be carried out using only the area-specific emissions rates of the building product as determined in test chamber measurements according to the AgBB scheme (see 4.1). Rather, it is necessary to additionally consider the indoor air situation likely to be encountered under practical conditions. The exposure scenario creates the link between product emission and concentration in indoor air. Thus, the evaluation must take into account the emissions from the product, the size of the room, the air change rate and the emitting surface area of the building product to be installed in the room.”

Unfortunately, planners and construction companies are always the “first” legal contacts in the event of complaints. They usually have to deal with the manufacturers. In turn, only very few manufacturers are prepared to provide truly comprehensive, prompt, and credible proof of emissions at the planning stage.

It becomes particularly difficult when several products are responsible for the elevated concentration levels at the same time. In these cases, it can happen that the “responsibility” and thus the liability “gets stuck” with the planner or the person carrying out the work.

Preventive recommendation

When selecting building materials, consider possible emissions and demand prompt, credible, and comprehensive test reports for harmful substances from the manufacturers of composite wood materials.