Environmental Toxicology Diary

There is a means to find out the association between a specific causative agent or suspected toxic chemicals with the observable effect by calculating the prevalence and incidence rate. Prevalence rate is a measure of the number of individuals who have particular attribute relative to the total number of individuals being observed. On the other hand, incidence rate is a measure of the new cases emerged during a specific time of study relative to the total number being at risk exposed by a suspected causative agent.

Being directly to the example, suppose you would like to know the association between working in the street with the development of lung irritation compare to those workers in mining within 1 year. You observe, say 500 individuals, at the end of the year you do the check up for those workers in the hospital for determining the developement of lung irritation.

Suppose there are 5 workers develop lung irritation over 500 individuals, then prevalence rate would be 5/500 = 0.01. Usually the terms is changed to 10 cases can be expected for every 1,000 individuals working in the street (for example traffic police officer). Now in other condition, let us assume, there are 8 workers have lung irritation relative to 500 individuals work in mining coal field. Thus the prevalence rate is 10/500 = 0.02, there is high chance that every 1,000 workers in the mining, there would be 20 people got lung irritation. What this result gives you is that you could compare these number and get brief conclusion that it seems working in the mining give higher chance occurence for individuals to develop lung irritation.

Now lets discuss about incidence rate, continue from above story, working in the street. Suppose you want to know new cases of workers in the street in year two to confirm the prevalence rate. Assume that there are 4 new cases were being discovered during the second year. The incidence rate would be 4/490 = 0.008, 8 cases may occur over 1,000 population working in the street. So 4 is the new case, new individuals who developed lung irritation, and 490 is people at risk, exposed in the street by pollutants, radicals. The number 490 derived from initial 500 people substracted with 10 individuals who have already had lung irritation. How about prevalence rate after two years. Well, that would become 14/500 = 0.028, 28 cases over 1,000 population.

So, basically the formula of prevalence rate and incidence rate are:

PR = number of people with effect during a specific time interval / total number of people
IR = number of new case in fixed period / total number of people at risk in the same period

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