Encyclopedia of Environmental Science and Engineering, Volume I and II

INDUSTRIAL HYGIENE ENGINEERING 517

patterns are required to assess ventilation requirement, the

previous formulae being inappropriate.

The Exposed Population

Where the hazard cannot be controlled at source or is not

effectively controlled during the transmission stage then it

is necessary to examine the exposed population in order to

engineer an environment around them which will minimize

their exposure.

This can be done in three ways:

1) released into still air by air displacement so that

they are bathed in an uncontaminated airstream,

2) by the use of conditioned refuges or enclosures,

3) by the provision of personal protective clothing.

Displacement Correct selection of airflow patterns can

serve to reduce the flow of airborne contamination in the

breathing zone. Displacement depends upon the creation of

a diffused clean air flow over the worker and towards the

work, thus carrying away the products of the work, prefer-

ably towards some form of extraction system. The provision

of work station supply ventilation also has corresponding

economies in the volumes of air required, as compared to a

system ventilating the whole building. Thus, displacement at

the work station can help to reduce the high cost of heating

replacement air during winter months. However, care must

be taken to ensure that local turbulence is minimized so that

the effectiveness of the control is maintained. This tech-

nique is most suited to well defined work stations. Working

posture and stance also needs to be carefully considered to

ensure success.

Thermal comfort of the worker must be considered so

that the combination of air temperature and velocity is such

that cold draughts are not experienced at the work station.

To this end the air supply diffusers must be carefully chosen

and the supply air temperature accurately controlled to suit

the air velocity at the worker. This is particularly important

where air is discharged from above and behind the worker

as the back, the neck and the back of the head are parts of

the body most sensitive to cold draughts. It should be noted

that air at a temperature above normal room temperature can

feel cold if it is flowing at a sufficiently high velocity. Charts

showing the relationship between air velocity and tempera-

ture and the part of the body affected (Houghton et al. , 1938)

in a room at 24C are shown in Figure 3.

The relationship is expressed as the number of people

complaining of discomfort as a percentage of the number

tested. As an example from Figure 3 it can be seen that a

draught velocity of 0.2 m/s at a temperature of 1.0C below

room ambient of 24C blowing on occupants’ necks will give

a feeling of coolness and will not be acceptable to 20% of

the room occupants whereas the same draught blowing on the

ankle region will discomfort only about 9% of the occupants.

Conditioned Refuges Isolating the worker from an uncon-

genial environment is a technique which is often adopted

where the process being worked is too large or too expensive

to control locally. Isolation cubicles can be used to protect

from noise, ionizing radiation, heat and cold as well as from

airborne toxins. in most cases the enclosure will require ven-

tilating and possibly air conditioning and the amounts of air

to be supplied will require some calculation. As a general

rule each person enclosed will require 10 litre/sec of fresh air

but this amount can be varied depending upon the size of the

enclosure and whether smoking is permitted or not.

If air conditioning is required the airflow rate will gener-

ally be in excess of that required for fresh air alone and some

recirculation will be normal. The volume of air required for

air conditioning will depend upon the heat gain to the space

and can be calculated from the expression:—

Q

S

t

12.

/



ms^3

where

S 
 the sensible heat gain to the room in kW

∆ t 
 the difference in temperature between the room

air and the supplied conditioned air C.

AIR VELOCITY m/s

AIR VELOCITY m/s

TEMPERATURE DIFFERENCE °C

0.1

0.2

0.3

0.4

0.5

0.1

0.2

0.3

0.4

0.5

–3 –2 –1 (^012)
TEMPERATURE DIFFERENCE °C
–3 –2 –1 (^012)
10%
40%
30% 40%30%
20% 20%
FEELING OF
COOLNESS
FEELING
OF WARMTH
FEELING OF
COOLNESS
FEELING
OF WARMTH
ANKLE REGION NECK REGION
10%
FIGURE 3 Percentage of room occupants objecting to draughts.
C009_003_r03.indd 517C009_003_r03.indd 517 11/18/2005 10:31:25 AM11/18/2005 10:31:25 AM