Wood Dust Exposure: Is an old material still giving problems today?

As Frank Lloyd Wright, an American architect, designer, writer, and educator said “wood is universally beautiful to man. It is the most humanly intimate of all materials.” Wood has been used for thousands of years for fuel, as a construction material, for making tools and weapons, furniture, and paper. As of 2005, the growing stock of forests worldwide was about 434 billion cubic meters, 47% of which was for commercial purposes. Dominant use continues to be for assorted items of furniture and extensively in building construction. Timber, also known as lumber in the US and Canada, is a term that encompasses a wide variety of woods that are available as processed or rough material such as planks and beams and is divided into two separate groups, namely hardwoods and softwoods. Hardwoods are timbers from deciduous trees, including trees from both temperate and tropical zones such as beech, ash, oak, mahogany, and teak while softwoods are mainly from coniferous trees such as Scots pine, yew, and cedar.

Unfortunately, dust is often generated by the machining and working of wood and wood-containing materials such as chipboard and fibreboard. Operations such as sawing, turning, and routing produce relatively coarse dust, while sanding and assembly operations generate finer dust.

It is worth pointing out here that dust particles can reach distinct parts of the respiratory system depending upon their aerodynamic diameter, a term used to describe their size. According to Cherrie et al ^[1], the aerodynamic diameter is the diameter of a spherical particle with a density of 1000 kg/m3 that settles at the same speed as the particle in question.  Particles, in reality, are irregular in shape and rarely spherical or circular, thus in this way, any irregularly shaped particle can be assigned an aerodynamic diameter and how far they reach within the respiratory system, particularly the lungs, depends on this diameter. When measuring particles, it is normal to select a size range of airborne dust because of the known importance of size in determining health effects.  These sizes or fractions are termed as the inhalable fraction, thoracic fraction, and respirable fraction as follows:

- The inhalable fraction, which includes the thoracic and respirable fractions is defined as the mass fraction of total particles that are inhaled through the nose and/or mouth:
- The thoracic fraction, which includes a respirable fraction, is defined as the mass fraction that penetrates the respiratory system beyond the larynx: and
- The respirable fraction is defined as the mass fraction that penetrates to the unciliated airways of the lung, known as the alveolar region, where the gas exchange takes place

And, while wood itself is indeed “beautiful”, according to the UK HSE ^[2] wood dust can cause serious health problems; a view shared by OSHA in the US ^[3] and Canada, where CAREX ^[4] estimates that approximately 304,000 Canadian workers are exposed to wood dust in their workplaces.  In the period 2002-03, around 3.6 million workers in the EU, circa 2% of the workforce, were exposed to wood dust making it a significant workplace exposure.  According to the National Cancer Institute (NCI), the situation is similar in North America.  The NCI state that occupations with high exposure to wood dust including sander operators in the transportation equipment industry, press operators in the wood products industry, lathe operators in the furniture industry, and sander operators in the wood cabinet industry. According to the US Department of Labor, overall employment of woodworkers is projected to grow 8 percent from 2020 to 2030, about as fast as the average for all occupations although employment of carpenters is projected to grow at only 2 percent over the same period, slower than the average for all occupations. Despite limited employment growth, about 89,300 openings for carpenters are projected each year, on average, over the decade. Most of those openings are expected to result from the need to replace workers who transfer to different occupations or exit the labor force, such as to retire.  It is ironic that it is often during retirement that chronic symptoms of a disease show themselves.

Exposure to softwood dust may cause symptoms and disease in the skin, eyes, nose, and airways. It can cause asthma, which carpenters, and joiners, for example, are four times more likely to get, compared with other UK workers. Furthermore, wood dust is classified by the International Agency for Research on Cancer (IARC) as a carcinogen, particularly for cancers of the nasal cavities and paranasal sinuses.

The Scientific Committee for Occupational Exposure Limits of the EU stated that exposure to wood dust above 0.5mg/m³ of inhalable dust induces pulmonary effects and thus should be avoided. Levels vary in different industries with mean levels in sawmills between 0.2 and 3.6mg/m³ for inhalable dust. In the furniture factories and joinery shops, it has been estimated that 87,000 furniture workers in the EU (12%) may be exposed to a level exceeding 5mg/m³, the occupation exposure limit (OEL) in the EU.

IOSH reported in 2016 ^[5] that a planned reduction was part of the EU’s proposal to amend the Carcinogens and Mutagens Directive (2004/37/EC) to limit the exposure to 13 cancer-causing chemicals and substances in the workplace by including new or amended limit values.  It stated that if the proposals were carried through, the EU's current OEL for hardwood dust would drop from 5mg/m³ to 3mg/m³ and the UK (then still part of the EU), along with 17 other countries including Hungary, Italy, Latvia, Lithuania and Romania, had to lower its limits by 2mg/m³.  The UK has indeed reduced its limit value accordingly in its January 2020 revision to guidance contained within EH40 ^[6].

OELs for hardwood dust in some member states were already well below the proposed (and current) limits and therefore would not be affected. In Denmark and France, exposure to this carcinogen is limited to 1mg/m³, while in the Czech Republic, Denmark, the Netherlands and Sweden it is 2mg/m³.  In the US, OSHA recommends an eight-hour exposure limit of 5 mg/m³ for both hard wood and soft wood with the exception of red cedar wood dust, for which the eight-hour limit is 2.5 mg/m³ due to its potential to cause allergic reactions.

A Swedish study ^[7] found that a determinant of exposure was cleaning with and without compressed air, maintenance work, indoor work, manual wood processing and sanding while factors that have been shown to decrease the exposure are, work in control rooms and efficient local exhaust ventilation (LEV). They reported some trends showing a decrease in wood dust exposure, but that exposure can still be high especially in new sub-sectors like the wood pellet industry. It also stated that it is important to measure the wood dust exposure in different industries since they can vary widely.

Direct reading, real-time devices can be extremely useful for preliminary walk-through surveys and/or for checking LEV performance. However, quantitative methods for measurement are described in documents like MDHS14/4 in the UK ^[8] and OSHA PV2121 in the US ^[9].  In both cases, they involve a gravimetric method that uses a personal sampling pump to draw air at a known flow rate, an appropriate sampling head for the size fraction of interest (total, inhalable, or respirable) and a filter that is weighed both before and after exposure. And since the sampling pump is designed to maintain a constant flow (even when the filter increasingly gets loaded), the concentration can be calculated in terms of mass per unit volume (g/m³) and compared with the OEL.  There are differences between the UK and US methods in the choice of sampling head, filter diameter and material based on historical custom and practice but essentially, they both use a common flow rate of 2L/min. It is important that the pump itself meets ISO 13137 ^[10] in several areas including flow stability and pulsation, which could otherwise mean a potential underestimate of the concentration.

France has one of the toughest limit values and the FCBA Technical Institute for the woodworking industry, was tasked with reassessing available direct reading methods because many companies were failing to meet the exposure limit, but it was felt that there was an unacceptable financial burden on small companies due to the need for extensive testing. However, earlier studies had not been able to satisfactorily establish equivalency with gravimetric methods, but an empirical relationship has now been established after widespread trials for the Casella Microdust Pro in conjunction with the Apex2 sampling pump, as a real-time means to demonstrate the effectiveness of controls.

The use of wood in its many forms will continue globally as a plentiful, sustainable material but that means a requirement for ongoing gravimetric monitoring and the (real-time) testing of control methods for the foreseeable future.

References:

1.       Monitoring for health Hazards at Work, 5^th Edition.   Cherrie, J, Semple, S and Coggins, M

2.       Woodworking health topics – Inhaling wood dust (hse.gov.uk)

3.       Wood Dust - Hazard Recognition | Occupational Safety and Health Administration (osha.gov)

4.       Wood Dust - Occupational Exposures - CAREX Canada

5.       EU proposals would slash UK hardwood dust limit | IOSH Magazine

6.       EH40/2005 Workplace exposure limits, 4^th Edition, January 2020

7.       Hagström, Katja & Schlünssen, Vivi & Eriksson, Kåre. (2016). Exposure to Softwood Dust in the Wood Industry. 10.1016/bs.coac.2016.02.017.

8.       MDHS14/4 General methods for sampling and gravimetric analysis of respirable, thoracic and inhalable aerosols

9.       OSHA PV2121, PARTICULATES NOT OTHERWISE REGULATED, TOTAL AND RESPIRABLE DUST

10.    ISO 13137:2013 Workplace atmospheres — Pumps for personal sampling of chemical and biological agents — Requirements and test methods