Aside from construction, wood is also used in interior design and cladding. One often hears that log buildings are particularly healthy, but actual scientific studies on the health effects of wood buildings are hard to find. A recent dissertation looked into wood and health, focusing on the microbiological properties of wood.
The hygienic properties of surface materials are important, particularly in hospitals, retirement homes or day-care centres, where illnesses can spread when pathogen-laden hands come into contact with surfaces that are then touched by other hands. People in these environments are often particularly vulnerable to infection, either because of a compromised immune system or, as in daycare centres, because of a habit of putting their hands in their mouths. Earlier publications on cutting boards had already identified the antibacterial properties of certain wood species, but the underlying cause of these properties was still unknown.
The studies compared different wood surfaces to glass, which is a very neutral surface that has no affect on the lifespan of bacteria on its surface. The wood species in the comparison were pine and spruce, with surface and heartwood examined separately. The bacteria on the surface of all the wooden test samples examined died more quickly than on the glass surface. Heat-treating the wood and removing organic compounds from the wood reduced the antibacterial effect, but did not completely eliminate it. Pine had almost without exception more of an antibacterial effect than spruce. This might be explained by the higher concentrations of organic compounds in pine, but the organic compounds themselves were also compared in an extracted form. Pine proved more effective than spruce, even as an extract. In many cases, pine heartwood had more of an antibacterial effect than surface wood; however, surface wood was more effective on E. coli in several tests. E. coli is a Gram-negative bacterial strain, unlike most of the other bacteria studied. The surface structure of Gram-negative bacteria is different from the surface of Gram-positive bacteria. This is why it may be affected by mechanisms different from those which affect Gram-positive bacteria. Generally, naturally occurring antibacterial agents are less effective on Gram-negative bacteria than on Gram-positive bacteria, which was also true in these studies.
The organic compounds explained the antibacterial properties of wood, as could be expected. They even proved effective on hospital bacteria such as MRSA and VRE. However, E. coli was not affected by anything other than pine surface organic compounds. Unfortunately, the organic compounds of spruce surface wood were not studied because it is more difficult to separate heartwood from surface wood. Because the differences in the chemical composition of pine and spruce heartwood and surface wood are significant, conclusions cannot be drawn about the antibacterial properties of spruce wood surfaces on the basis of heartwood results. Lignin was another component found to have antibacterial effects. Cellulose, on the other hand, and hemicellulose in particular, increased the growth of bacteria rather than decreasing it.
No single substance in the organic compounds was found that would have explained their antibacterial properties. On the contrary, the tests seem to show that a suitable combination of different substances underlies the antibacterial properties of wood. In addition, the results varied depending on how the tests were conducted: in liquids, the antibacterial properties of pine surface wood were better than heartwood, while the organic compounds of pine heartwood had more of an antibacterial effect than surface wood on dry surfaces.
Volatile organic compounds (VOC) emitted from wood do not actually pose any health hazards, but because many VOCs from different sources are harmful, there has been discussion about reducing VOCs in wood. However, many sufferers of allergies feel that wood buildings are the healthiest form of housing for them. Scientific research has major shortcomings in determining what qualities and in what amounts indoor VOCs could increase the well-being of people living amongst them. This dissertation examined the antibacterial effect of VOCs on bacteria that cause illness. In the study, the VOC concentration was higher than in any wood building and the results were rather preliminary and indicative. The greatest antibacterial effect was from VOCs of pine heartwood, which were also present in the greatest quantity. Other tree species studied also had some impact. α-pinnate and limonene, which were examined separately, also had antibacterial properties. There is still plenty to study in the antibacterial properties of wood. Based on the results of this particular study, emerging themes of research and product development include the creation of new products with extracted organic compounds and lignin and the use of untreated wood surfaces or the purposeful treatment of surfaces to make them more hygienic. The potential benefits of VOCs are also well worth researching, as they could lead to interesting results for the wood industry. Resistant bacteria are a growing problem, and no stone should be left unturned in the search for compounds that could curb them.
Text: Tiina Vainio-Kaila
Tiina Vaino-Kaila’s thesis, “Antibacterial Properties of Scots pine and Norway spruce” can be found at: https://aaltodoc.aalto.fi/handle/123456789/28650