Epidemiology for Engineers

Epidemiology is the study of disease through the use of statistics. Statistical data about diseases can illustrate the mechanisms of transmission, highlight good health practices and bad hygiene, assist the economic evaluation of risks/benefits, and often tells a more complete story than clinical or biological analysis. The data summarized graphically in this study has been selected to enable engineers to make informed decisions regarding the design of buildings and ventilation systems, as well as to enlighten the general public about the nature of respiratory disease transmission.

SOURCE OF COLD INFECTIONS IN ADULTS

COLD INFECTIONS IN ADULTS

The limited data available indicates that most cold infections are acquired in the office or work environment. The home environment provides the second largest source of infections. The Figure shows some 51 % unknown, which could also be due to home and office infections, or any enclosed locations where prolonged exposure to infective individuals could occur, such as bars, dance halls, restaurants, churches, other people's home, indoor sporting events or theaters.

One detailed case study performed in an isolated Alaskan community provides some epidemiological insight into this matter. Five individuals in the town of Klawock had attended a conference in Ketchikan where they had been exposed to influenza B. Shortly thereafter the flu spread through 33 of the 37 households in the town. Of the 181 residents in Klawock, 150 contracted the illness and one person died in the 4-week course of the epidemic. The attack rate was 83 %. Normal attack rates for influenza in cities are about 20 - 40 %.

RISK OF ACQUIRING A COLD AT HOME

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RISK OF ACQUIRING A COLD AT HOME & AWAY FROM HOME

Some inferences can be made from this chart. Firstly, infants are highly susceptible, and children bring home most of the colds from school. The wife is twice as likely to catch a cold at home because she cares for the children at home.

ADULT INFECTION RATES AT HOME -- w/ & w/o CHILDREN

From the above chart it can be seen that the children and infants are most susceptible, and that the housewife who cares for them is likely to catch the cold also. Nowadays, whichever parent stays at home to care for the kids would be more likely to catch a cold.

RHINOVIRUS TRANSMISSION RATES IN HOUSEHOLDS

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The above chart shows the marked difference between transmission rates of someone in an infection stage, and when they are out of the infectious stage. The presence of rhinovirus on the hands and saliva suggests that transmission might be by direct contact instead of airborne. On the other hand, it may be reflective of cold symptoms only.

ROUTE OF INFECTION OF COLDS

ROUTE OF INFECTION OF COLDS

The results displayed in this chart show that the nose and the eyes are the most vulnerable routes of virus invasion. The hand-to-mouth route could not be demonstrated to play any part in cold infections, although several investigators have insisted this is the primary route. The route of hand-to-eyes was, unfortunately, not studied. In spite of the limited data on which this chart is based, no subsequent studies have revoked these estimates and several have provided corroboration.

LOCATION OF RHINOVIRUS IN PEOPLE WITH COLDS

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The predominance of rhinovirus on the hands shown above may reflect the fact that transmission by hand contact is a primary mode of infection.

SOURCE OF COLD VIRUS DISPERSION

This chart provides perspective on the source of airborne viruses. Sneezing and nose blowing provide the overwhelming majority of aerosolized viruses, and from these come the infections. The risk of contracting a cold from someone who is coughing in the room seems relatively minor, while the risk of getting infected from talking to a person with a cold is negligible.

COLD VIRUSES -- Route of Infection

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MEDICAL COST OF DISEASE AND LOST EARNINGS

The breakdown of medical costs provides an indication of the proportion of respiratory diseases, although not all of these are of the infectious type. Of greater interest to corporate managers is the work time lost to respiratory disease and the resulting lost earnings. Inference suggests that respiratory infections are the largest cause of lost earnings that might be reduced by the control of indoor transmission.

PER CAPITA MEDICAL COSTS -- 1980

PER CAPITA MEDICAL COSTS

More readily understood by the average person, the per capita medical costs indicate no more than 10% is spent on respiratory disease.

BREAKDOWN OF RESPIRATORY INFECTIONS

BREAKDOWN OF RESPIRATORY INFECTIONS

This chart comes from a detailed study of the etiological causes of respiratory infections. The general category of Upper Respiratory Infections (URI) includes the common cold, which can result from different viruses. Colds clearly comprise the largest single respiratory infection, although during flu epidemics, influenza will predominate.

THE EPIDEMIOLOGY OF AIRBORNE TRANSMISSION

The basic equation (1) that describes the statistics of epidemics is:

C = r I S

where C = number of new infections
r = average contact rate
I = number of infected disseminators
S = number of susceptible individuals

Standard Model -- Non-Epidemic Conditions

The production of new infections in a susceptible population is termed a generation. Several generations typically comprise an epidemic, as the infection is constantly re-transmitted to new individuals. Once the supply of susceptible individuals has been exhausted the epidemic must end. Normally, the epidemic will end before everyone is infected because there will be insufficient population to propagate the epidemic. That is, the density of susceptibles becomes so low that no new transmissions occur. Mathematically, if C/I > 1.0 it denotes a propagating epidemic. If C/I < 1.0 the epidemic will rapidly fizzle out.

In a rearrangement of equation (1):

C/I = r S

Wilson Model --Non-Epidemic Conditions

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The value rS determines the value of C/I and is called the contagious potential. The rate r can only be determined by epidemiological data, but a value of between 0.1 and 0.2 would be typical and conservative for most respiratory infections.

This equation can be adapted for infections spread by ventilation systems by defining these terms:

i = quanta of infection produced by each infective individual
V = volume of ventilation air
s = volume of air breathed by each susceptible individual

Based on these values, the following products can be defined:

iI = total amount of biological contaminant added to air
sS = total volume of inhaled contaminants

The modified equation becomes:

C =

Rearrangement yields (2)

C =

Comparison with equation (1) makes it clear that the contact rate will be

r =

Ventilation Model --Non-Epidemic Conditions

Normal breathing determines the value of s to be approximately 0.5 CFM per person. Also, since most epidemics are of the airborne variety, a value of r between 0.1 and 0.2 will still provide a conservative estimate.

Equation (2) then takes the form:

C =

The value of I depends on the particular virus or bacteria. For measles the value has been found from epidemiological studies to be 9.1.

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