In bulk, wood is unlikely to give rise to toxic effects. The hazardous forms that may give rise to health risks are:
Ill health effects associated with wood
Skin: The main effect is irritation. This can be caused by skin contact with the wood, its dust, its bark, its sap, or even lichens growing on the bark. Irritation can, in some species of wood, lead to nettle rashes or irritant dermatitis. These effects tend to appear on the forearm, back of the hands, the face (particularly eyelids) neck, scalp and the genitals. On average, they take 15 days to develop. Symptoms usually only persist as long as the affected skin site remains in contact with the source of irritation, e.g. wood dust. Symptoms subside when contact with the irritant is removed.
Sensitisation dermatitis is more problematic and is usually caused by skin exposure to fine wood dust of certain species. This is also referred to as allergic contact dermatitis and results in similar skin effects to those produced by skin irritants. Once sensitised, the body sets up an allergic reaction, and the skin may react severely if subsequently exposed to very small amounts of the wood dust. Cross-sensitisation may develop where other woods or even non wood materials produce a similar response.
Respiratory and allied effects: Wood, especially inhalation of fine dust, can have many effects on the respiratory tract, including :
The most common effects arise from irritation, where symptoms usually only persist as long as the sufferer remains in contact with the irritant. Allergic effects, as a consequence of sensitisation to wood dust can also occur, e.g. rhinitis.
Impairment of lung function
Rarely - extrinsic allergic alveolitis (a disease with 'flu-like' symptoms which can cause progressive lung damage), eg when using western red cedar, iroko.
Asthma is of particular concern. Most wood dusts can irritate the respiratory tract provoking asthma attacks in sufferers; although effective control of dust levels normally improves the problem.
Some wood dusts can cause asthma as a specific allergic reaction. Once sensitised, the body will quickly react of subsequently exposed, even to tiny traces of dust. Unlike irritation, where people can continue to work with the dust once it is controlled to below the level at which irritation occurs, people who become sensitised will not normally be able to continue working with the dust, no matter how low the exposed.
Whole body: Inhalation of some wood dusts can have general (whole body) effects,
eg, South African boxwood, although this is not usual for the common commercial woods. Many effects have been described including headache, thirst, nausea, visual disturbance, drowsiness, anaemia and hepatitis.
Toxic activity is specific to a wood species. Knowing the exact species is important in establishing what the potential toxic effects may be. Individual wood species (of which more than 100 are commercially important in the UK) are very easily confused.
For example, rosewood may be used for up to 30 different species; and an individual species may have up to ten different trade names. An additional difficulty is that trees vary within a species. One specimen may contain low levels of its toxic agent and the next contain much higher levels. So experience may not be a reliable guide.
Wood is classified into two broad families: Hardwood & Softwood
The classification is botanical and depends on the fine structure of the cells in the wood species. It does not refer to the physical properties of the wood. For example, balsa wood is a hardwood!
Users should remember that veneers are often made of hardwoods; so are composite materials such as plywoods. The type of wood making up particle boards, eg, chipboard, hardboard, MDF is not always known, but is usually a high proportion of softwood. However MDF sourced from areas of the world where hardwood predominates, such as Malaysia & Indonesia, will be more likely to be composed of a high proportion of hardwood.
The HSE have designated both hardwood and softwoods as Asthmagens and hardwoods as a Carcinogen.