In recent years a special disease named the wood trimmer's disease has been observed with increasing frequency among saw mill workers. Particularly workers in the trimming department have been affected. The medical symptoms are usually high fever, ague headache, pain in muscles and joints. It was shown that the disease was of immunologic nature and caused by over-exposure to spores from mould fungi. With the aid of common dust, pollen or spore samplers extremely high spore counts were sometimes observed in the air the trimming departments. These high spore counts were found to be correlated with the intake of timber packages with visible mould growth. Such mould infected dried timber packages were occasionally observed to have come from the drying kilns. The timber, following the production line, is usually transported with little delay from the kiln to the trimming section, where stickers are removed and the individual planks and boards are trimmed and quality sorted. So if there is mould on the timber when is leaves the drying kiln all this mould is also introduced into the trimming section. Substantial quantities of spores are then released with the relatively violent handling of the timber there. There are many different types of drying kilns in operation in Sweden. In certain types the newly sawn timber packages are placed in a chamber where the drying climate successively changes from high humidity to low humidity and where either the dry temperature or the wet temperature increases during the drying process. In other types the timber packages are transported in a long tunnel-like chamber where they meet a progressively changing climate. Generally the starting (dry) temperature varies between 35 and 40°C and the final temperature near the outlet of the kiln used to be 40-60°C depending on the type and construction of the kiln.·Simultaneously the relative humidity changes from about 100% to 30-60%. The average moisture content of the timber decreases during the kiln drying from usually > 100% for the sapwood and around 40% for the heartwood down to < 18%. It can be mentioned that more than 95% of the sawn timber in Sweden is from Scots pine (Pinus·silvestris L.) and Norway spruce (Picea abies L. Karst.). The gentle drying processes outlined above are necessary to obtain high quality timber for export. On the other hand the temperature range used permits the growth of thermophilic and thermotolerant mould fungi as long as the substrate (wood) moisture and the relative humidity are sufficiently high. As a consequence the kiln drying process is a delicate balancing act to find a climate that reduces checking, twisting, falling out of knots, resin exudation etc. to a minimum at the same time as mould growth is prevented. This balancing act is not always successful.

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