5. A housing estate of 250 people is considered to be at risk from a nearby fact

Question

5. A housing estate of 250 people is considered to be at risk from a nearby factory on the basis of a hazard analysis which yielded the following data: (i) The 20% probability of fatalities due to the blast effects of an explosion. (ii) The 10% probability of additional fatalities due to the release of toxic gas following the explosion. (iii) The probable frequency of an explosion occurring is once in ten thousand years, ie. 0.0001 yr. (a) Find the societal and respective average individual risks for these events (b) Explain why such data may be misleading if the number of people exposed to the risks is increased by, for example, the expansion of the housing estate. Teesside Usiversity Open Learning © Teesside University 201

Explanation / Answer

Since in the question it is given that the probability of fatalities due to blast is 20%

and the additiona;l prob of 10% fatalities due to toxic gases following the explosion so the total fatalities will be

20% (Due to blast) + 20%*10% (Due to toxic gas after blast) =20%+2% = 22%

While the frequency of explosion is 0.0001 per year

a) so the avg individual risk = probability*Frequency = 22% * 0.0001 = 0.0022% or 0.000022

And so the societal risk = People * avg individual risk = 250 * 0.000022 = 0.0055

Which shows that in a year out of 250 people the people at risk are 0.0055 which is negligible

b) Now if the expansion of housing estate will happen and so the number of people staying at the place will increse to say 100,00,000 then annualy the people at risk will be = 100,00,000 * 0.000022 =  220 and as the number has increased now it can be used as alarge figure to show that avg 220 people are at risk while if you analyse the real case the only probability of fatalities due to blast is just 0.0022% per year which is nothing but negligible but when the same is multiplied with number of people the number suddendly started looking bigger and it may mislead as the number in isolation without probabilities do not make any sense.

Hope this answer will make your concepts clear. Kindly upvote the ans if it has really helped you in understanding the problem. Good Luck!!

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