What Does Twa Mean for Exposure Limits?

In every industry, there are occupational health and safety guidelines to ensure the health and wellbeing of every individual. Workplace exposure standards for chemicals and harmful substances exist to reduce health risks to workers. From inhalation to absorption through the skin, and ingestion, exposure to mixtures and substances can occur.

The extent of exposure is determined by the chemical concentration in the air, the duration of exposure, and the effectiveness of controls. So, what are exposure limits? And what does TWA mean for exposure limits? Let’s unpack this below.

What Does TWA Mean For Exposure Limits?

Let’s start with the TWA definition. TWA stands for time-weighted average and is a standard based on the exposure to a chemical or substance within a period of time. Time-weighted average is one of the three types of exposure standards listed in the Workplace Exposure Standards for Airborne Contaminants.

What Are Workplace Exposure Standards?

Our bodies respond in different ways to exposure, depending on the amount absorbed and type of mixture. Toxic and harmful chemicals can negatively impact an individual’s health.

In Australia, guidelines have been established for approximately 700 substances and mixtures. These are known as workplace exposure standards (WES). TWA is included in these standards.

According to Safe Work Australia, the workplace exposure standards set out the legal concentration for a particular chemical that cannot be exceeded. They also identify airborne concentrations of a chemical that are not expected to cause adverse health effects to a worker.

When there is uncertainty about adherence to exposure standards or analysing a health risk, businesses may require airborne monitoring. Companies must keep these records for at least 30 years and have them available to exposed workers.

Airborne monitoring can also help test and decide which exposure controls are most effective or analysing how changes to a process or production method have affected exposure levels.

Time Weighted Average Standards

The time-weighted average provides standards for an 8-hour working day and a 40-hour workweek. When working hours exceed this daily or weekly amount, the standard can be adjusted to compensate for increased exposure during shifts and reduced recovery time.

In practice, the concentration of airborne contaminants fluctuates significantly; however, it is ideal to keep exposure limits below the 8-hour TWA exposure standard consistently. The standards account for short term excursions above the specific amount, provided there are extended periods of underexposure throughout the working day.

That being said, if a short-term exposure exceeds three times the TWA standard for more than 30 minutes per 8-hour day, it is not considered to be under reasonable control. Additionally, if a single short-term exposure exceeds five times the 8-hour TWA exposure standard, it is not considered under reasonable control.

Short Term Exposure Limit

When considering “what does TWA mean for exposure limits?” we also need to think about how other workplace exposure standards inform practices and procedures. (STEL) values, or peak limitation guide practices for short term exposure to airborne contaminants. This limit is the maximum time-weighted concentration of a substance over 15 minutes. Unlike the 8-hour time-weighted average, these standards are not adjustable, measuring adverse effects due to acute-over exposure.

Peak limitation controls the short-term exposure of airborne contaminants and is an important supplement to the eight-hour TWA. These limits are recommended where there is potential for adverse health effects from high short-term exposure. STELs aim to minimise the risk of intolerable irritation, irreversible tissue damage and narcosis that could increase the risk of workplace accidents. For STELs, there are a few key points that are essential to know:

• The exposure limit should never be exceeded
• Exposures at the specific limit should not be for longer than 15 minutes
• No more than four exposures at peak limitation should occur in a day
• Between successive exposures, there must be at least a 60-minute interval

Peak Limitation 

With the TWA definition and STEL standard, there is also peak limitation.  Peak limitation exposure standards regard the maximum airborne concentration of a substance over the shortest practically measurable period. This period must not exceed 15 minutes and is used for rapidly acting substances and mixtures, which are not appropriately measured using an 8-hour time-weighted average. Peak limitation standards represent the peak concentration to which workers can be exposed and must never be exceeded. Even with brief exposure, chemicals at high concentrations can induce adverse health effects.

Why TWA Hazmat Resources Are Vital

While these standards provide a solid grounding in managing risk from exposure, it is also important to note that hazardous materials and chemicals all behave in unique ways. The effects of these materials also differ from person to person, so steps to reduce risk as much as possible need to be taken. Exposure standard lists and occupational exposure limit tables are used to identify the unique effects of certain chemicals, including sensitisation, how they are absorbed through the skin, and the way they interact with other mixtures.

Talk to the Experts

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