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There are two concerns with respect to human interaction to the thermal
-Thermal stress and strain
Excessive thermal stress means that the body has to work hard to avoid he core
temperature moving outside narrow limits (37 + 2 C). If that happens we’re in a
serious situation which leads to serious health effects and may be fatal. Our efforts to
prevent this happening can also lead to adverse effects.
This is most likely to occur in extreme environments or, sometimes, in more
moderate environments where particularly heavy work is being performed or clothing
is worn which prevent metabolic heat escaping.
Thermal comfort is most likely to be an issue in workplaces such as offices, but
complaints or concerns can sometimes occur in manufacturing environments and
other types of workplace
It is also something that may need to be addressed in leisure facilities.
The first thing to note about “thermal comfort” that differentiates it from thermal
strain is that the body is not experiencing a level of stress that it can’t cope with.
There isn’t a physiological problem and ill health will not occur due to excessive
But that encompasses a wide range of conditions. Will all of them be “comfortable”?
Experience clearly shows that the answer to that is “no” !
So what is thermal comfort?
This is the definition given in ISO 7730, (more about that later)
So thermal comfort is a psychological or psycho-social issue
It’s about people’s opinions, and you can’t keep everyone happy all of the time.
Studies have suggested that the very best you can do is achieve 95% satisfaction .
So there will always be 1 person in 20 who is dissatisfied. Change things for him/or
her, someone else will start to feel unhappy!
In practice you’re doing well if more than 80% are happy!
But what we think is influenced by the environment and this is true of our perception
of thermal comfort. So if we want to understand what people consider to be a
“comfortable” state we need to understand how we interact with the thermal
Human beings create heat, so one thing we have to consider is how much heat we’re
generating That really depends on our activity level or, in an occupational setting, our
work rate. The more physically demanding our activity, the more heat we’re
Depending on circumstances we may lose heat to the environment, or we can gain it.
So there is heat exchange. There are a number of ways that this occurs.
Convection is one of the main ways that heat transfer between people and the
Radiant heat can be a problem in some cases. Everything gives of radiant heat – the
amount depends on the surface temperature. In thermal comfort situations it is unusual
to find hot surfaces that give off significant radiant heat. However, solar radiation can
be a problem in some cases, particularly in buildings such as offices with lots of
Evaporation is one of the main ways we lose heat.
We gain heat by convection if our skin temperature is higher than air temperature.
However, in most situations where there are complaints about thermal comfort the air
temperature will be lower than skin temperature and we are more likely to be losing
heat by this means
Radiant heat depends on the mean radiant temperature. This is the mean temperature
of all the surrounding surfaces including the floor, walls, ceiling and any objects
Radiant heat exchange is also affected by solar radiation.
Evaporative cooling depends on the air temperature and relative humidity
These factors influence our heat balance. But there’s another one that interferes with
the exchange of heat between the body and the environment
Clothing level has a significant influence on heat exchange. It interferes with
convective and radiant heat exchange and affects evaporative efficiency. Heavy
clothing can even influence metabolic rate (although that won’t normally be a
problem for thermal comfort related situations)
So if we are to assess thermal comfort thoroughly we need to quantify these four
And we also need to consider the “human factors” – metabolic rate and clothing.
Draughts cause local cooling and are likely to lead to complaints
Radiation asymmetry may occur where there is a directional heat source and can
often be associated with solar gain through windows
As hot air rises, in most buildings air temperature increases with distance above the
floor. The extent of this difference depends very much on the type of heating
employed. With radiators the temperature varies little with height, but with warm air
heating a significant temperature gradient can result. If the gradient is sufficiently
large, local warm discomfort can occur at the head and/or cold discomfort can occur
at the feet, even though the body may be thermally neutral
Floor temperature is not normally an issue in most workplaces but needs to be
considered in situations where people may have bare feet – e.g. Swimming pools,
gyms, changing rooms and the like
There are two basic approaches we can take to evaluating thermal comfort
-a Qualitative approach where we may not take any measurements but rely on other
forms of evidence
- a Quantitative approach where we carry out measurements. (However, interpreting
the results from the measurements presents some difficulties)
A quantitative assessment involves talking to and interviewing people in the
workplace and looking around for obvious clues, using experience to make a
The thing to remember is that “comfort” is subjective – one person may complain
about feeling warm while another may consider the same environment to be too
cool. So the best way to find out what someone thinks is to ask them!
There are a number of questionnaires and checklists available from various sources
which can help with this qualitative approach. Some examples follow (there are
others, or you can even devise your own)
This very simple checklist is available on the HSE website
This is a 5 point scale devised by Fanger (more about him later)
The BOHS Technical Guide on the Thermal Environment has a couple of examples of
questionnaires that can be used during a qualitative assessments.
This one is fairly simple and mainly addresses “whole body” comfort
This is a little more comprehensive. It allows evaluation of local discomfort.
If we’re going to undertake a thorough quantitative assessment we’ll need to quantify
The environmental parameters
-Mean radiant temperature
And two “human” factors
-Work rate (which affects metabolic rate) and
A standard thermometer measures air temperature.
The traditional way of measuring relative humidity is to use a “whirling” or “sling”
hygrometer. It has two thermometers – one of which has it’s bulb covered with a
dampened muslin wick. The difference in the readings obtained is proportional to
relative humidity which is determined using a psychometric chart or special slide rule.
Electronic humidity meters are available and may be less embarrassing to use! But
they will need calibrating.
Radiant heat can be assessed using a globe thermometer.
Note that it does NOT give a direct reading of mean radiant temperature as it is
affected by both radiant and convective heat exchange. The reading is known as the
“globe temperature”. If the air temperature and velocity are also known, then it is
possible to calculate the mean radiant temperature/
This is a smaller version of the globe thermometer. Note that it is less accurate the
The difficulty with measuring air velocity is that in most situations where we need to
assess thermal comfort it is relatively low and omni-directional which makes
The kata thermometer does not measure temperature but the time it takes to cool is
determined and from this, if the air temperature is also known, the average air
velocity can be estimated.
Various electronic instruments, such as this, are available. Remember that they will
need to be calibrated from time to time
Metabolic rate can be estimated using standard tables found in ISO 7730 and other
Like metabolic rate, clothing level can be estimated using standard tables found in ISO
7730 and other texts.
1 Clo represents the insulation provided by an “American business suit”
Now we have quantified six parameters what do they mean? We need to find a way
of pulling them together so we can decide whether we have a problem and, if so,
how big the problem is.
The PMV / PPD index is widely use when assessing thermal comfort.
It was developed by Professor P O Fanger who conducted extensive research into
thermal comfort using a large number of subjects both in Denmark and the United
It forms the basis of the International Standard ISO 7730
The first step is quantify the main factors that affect thermal comfort and then to
determine the predicted mean vote (PMV) .
The PMV is then used to determine the predicted percentage dissatisfied (PPD).
This Fanger’s equation, develoed from his research
As you can see it is very complex and quite unsuitable for manual calculation.
However, ISO 7730 contains a BASIC programme which can be used to obtain the
PMV and PPD. There is also a series of charts, derived using the equation which are
relatively easy to use.
BS EN ISO 7730
A quantitative approach to evaluating thermal comfort that uses Fanger’s PMV PPD
Also provides guidance for the design of a comfortable thermal environment with
recommendations for both whole body and local thermal comfort conditions.
It provides a computer programme (in Basic) and tables, either of which can help you
determine the PMV and PPD for a given situation
The tables do not cover all possible conditions but allow a reasonable estimate to be
When using the tables you need to determine the operative temperature.
At low velocities this is the mean of the air temperature (dry bulb) and mean radiant
temperature (NOT the globe temperature). As the globe thermometer responds to
both convective and radiant heat the globe temperature will be close to the operative
temperature at low velocities.
So, which is the best approach to use when people are complaining about feeling too
hot or cold?
Well, when investigating a problem its probably best to use a combined approach
In my view the first think to do is to talk to people to find out what they think and
look around for obvious problems. Thermal comfort is subjective so if people are
complaining there’s a problem even if using the PMV/PPD index predicts people will
be generally happy. If they’re unhappy, something needs to be done.
In order to decide on solutions we need to find out why we have a problem. Quite
often this can be determined by looking around and talking to people. However this is
where carrying out measurements can help. They can help you to determine which of
the 6 main factors are likely to be causing the problem