What is a Workplace Exposure Standard - WES and TWA?
A workplace exposure standard (WES) is a New Zealand Department of Labour standard. It defines what the New Zealand Labour department considers is a concentration of a substance that is designed to protect the worker from the effects of long term exposure.
The unit used in the standard is milligrams of a substance per cubic meter of air (mg/m3)
The standard often relates to an exposure over a period of time. Typically one shift either 8 or 12 hours. The standard for one shift is referred to as a “ Time Weighted Average” or TWA.
Change to the Limits
In December 2010, the Department of Labour lowered the Workplace Exposure Standard (WES) for softwood dust
The new WES for softwood dust (effective from December 2010) is: 2mg/m3 TWA for both eight-hour and 12 hour shifts. The previous work exposure standard (wes) for softwood dust was: 5mg/m3 TWA (Time Weighted Average)
The new WES will be implemented over a two-year period, commencing 13th December 2010. This means that employers have two years to make any changes necessary to meet the new WES.
Potential Health Effects of Wood Dust
Wood dust encompasses a variety of woods, wood dust can contain:-
- a wide range of chemical constituents (eg resin acids).
- a range of microbes (bacteria, fungi) growing on the wood,
- chemicals from treatment of wood
Each of the above has the potential to have health effects. Wood dusts can stay in the body far longer than a gas or vapour such as formaldehyde. The adverse effects caused by wood dusts can be irritant, allergic, and have carcinogenic effects.
Why a Lower Softwood Dust Limit?
Current research shows that adverse health effects of softwood dust can occur at concentrations lower than previously thought. Lowering the WES value means more may have to be done in the workplace to reduce the level of worker exposure to this substance.
Change to Limits
Overseas agency’s have lowered their limits to 1 mg/m3. This will reduce the risk, not necessarily eliminate it. The adverse health effects of wood dust are not just nasal, but also impaired lung function and respiratory symptoms can occur. Some studies suggest some adverse health effects may occur at levels below 1mg/m3.
Health Effects of Softwood Dust Exposure?
The effects of exposure to softwood dust include:-
- upper respiratory tract irritation such as a cough,
- nasal irritation
- eye irritation,
- blocked or runny nose,
- sinus infections and
- long-lasting colds.
Other effects may include –
- changes to the cell lining of the nose and other respiratory mucosa,
- a decrease in the ability to clear fluid from the upper respiratory tract.
- Adverse effects may also be found in the lower respiratory tract, including impaired lung function,
- occupational asthma,
- bronchitis, scarring of the lung tissue, and
- other allergic respiratory problems.
Cancer Causing
Some wood dusts are carcinogenic to humans. Certain types of hardwood such as beech and oak are associated with cancers of the nose and respiratory tract. However, softwood dust also poses potential for cancer development.
In some industries, workers are exposed to both hardwood and softwood dusts, making the lines of distinction between the health effects of hard-and softwood dust more blurred.
Occupations with Softwood Dust Exposure?
People are usually exposed to softwood dust in the following industries:
- furniture and other wood manufacturing
- joineries
- carpentry
- wood processing,
- including sawmills
- veneer plants
- plywood plants
- wood chip operations
- medium density fiber board operations
Sensitisation
This is where the body reacts to a toxin at an ever decreasing level.
If you develop an allergy and become sensitised to wood dust, it can be very difficult to control exposure down to a level where the allergy is not triggered. Lowered levels may not help those workers who have already developed such an allergy.
Air Sampling for Softwood Dust?
Special monitoring equipment is needed to measure airborne substances in the workplace (called air sampling).
The equipment can cost a lot of money. In most workplaces, air sampling is ideally carried out by people who are:-
- Specifically trained to do so,
- Have qualifications to do so,
- Are from an organization that is ISO 17025 accredited.
In all instances WES relate to exposure that has been measured by personal monitoring using methods that gather air samples from the workers’ breathing zones. The objective of personal monitoring is to assess the air a worker is breathing.
A size selective sampling head holds a pre weighed filter. Air is drawn through the filter over a period of a shift. The filter is recovered after sampling and a result is calculated.
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