Occupational Hygiene and Ergonomics
HAZARD CONTROL EXERCISES 2020
(Three questions, a total of 15 marks)
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Question 1 (Respiratory Protection, 7 marks) (show your working)
Remembering the previous exercise on chemical hazards, you are now going to recommend the use of respirators to reduce the exposure of the workers:
Background:
You are a health and safety consultant to a company manufacturing fi
eglass (fi
e-reinforced plastic) shower cubicles for the mining industry. The process involves a number of steps. A chemical called styrene is present in the resin, which is applied with a
ush.
The following YouTube video gives you some idea of the process, if done manually. https:
www.youtube.com/watch?v=BOo8gxp3K3w
(especially the section at 3 minutes onwards)
A group of employees is required to work in a workshop area where the average air concentration of styrene vapour has been measured to be 50 parts per million (ppm), with some variability during the day.
timeAir concentration of styrene
8 hour average standardBrief high exposure during cleaningShort term exposure standard
You recommend the use of a ru
er (non-disposable) half-face respirator with a "nominal" (manufacturer-listed) protection factor (NPF) of 10.
If used properly, what would be the predicted concentration of styrene inside the respirator?
(1 mark, show your working)
Hint: The protection factor is the ratio of the concentration outside of the respirator to the concentration inside the respirator. See the notes on respiratory protection(p10)
The three main determinants of respirator effectiveness are filter efficiency, facial fit and wear time.
One of the employees in the abovementioned environment works for eight hours, but is observed to wear his respirator for only 80% of the time. What would be his expected time-weighted-average inhaled concentration of styrene?
(1 mark, show your working)
Is the fact that he is not wearing his respirator all of the time a problem in this case?
(2 marks)
Hint: See the definition of TWA (time-weighted-average) on page 1 in the Basic Science Concepts
Time weighted average, the "average" value over a given time period, usually eight hours. For example if a person was exposed to 20 ppm benzene vapour for 1 hr, 60 ppm for 3 hr, and 100 ppm for 4 hr, the TWA exposure would be ((20x1) + (60x3) + (100x4))/8 = 75 ppm.
The so called "workplace" protection factor (WPF) is a measure of the protection in the actual working conditions. It is usually difficult to assess the WPF, but one way of determining the WPF is to measure the concentration of an air contaminant inside the respirator, and compare it to the co
esponding air concentration outside the respirator.
Suppose this was done for the workers wearing the respirators in the workshop above.
The WPF was found to be 5 but the NPF for this respirator is 10. In other words, the concentration of styrene inside the respirator was greater than expected (twice as much). What factors might be contributing to less protection?
(3 marks)
Hint: Consider the various parts of the half face respirator in the diagram above, and human factors.
Question 2 (Chemical protective clothing, 3 marks)
Preliminary remarks:
Gloves are often used to prevent skin absorption of chemicals. Choosing the right type of glove requires an understanding of the chemical, but also the task etc. The question challenges you to think of all the factors that might be relevant in making a practical choice.
Using the glove permeation charts (and other information) provided as resource material for the topic of chemical protective clothing, which glove would you recommend for a worker required to do extensive cleaning up of metal components with turpentine?
(1 mark)
Justify your choice.
(2 marks)
Image (http:
www.sca-tork.com/)
Hint: consider more than just resistance to permeation.
Question 3 (Ventilation, 5 marks)
Preliminary remarks:
Local exhaust ventilation (LEV) is a form of engineering control for ai
orne chemical hazards and heat. It can be a very effective long term solution. However, the design and ongoing management of such systems does require careful consideration. This question asks you to consider a range of factors (including management) that determine their effectiveness.
(a) Consider an arc welding process and an associated fume capturing hood.
(i) Explain, perhaps with a
ows on the diagram, the difference between hood face (entry) velocity and capture velocity. (1 mark)
(ii) Which of the two is the most important? (1 mark)
(b) Outline some of the issues that influence the effectiveness of local exhaust ventilation as a long-term means of control of air contamination. You can choose a particular workplace, e.g. foundry, electroplating facility, furniture workshop etc., to illustrate your answer.
(3 marks)
Hint: Imagine that you are the occupational health and safety manager, and an auditor has asked a question about how your company ensures that the ventilation systems are effective.