Many designers appreciate that thermal comfort has a significant impact on productivity but they are rarely taught why that is the case. This paper sets out the (often under-appreciated) extent to which body performance is affected by blood flow which in turn is affected by the immediate thermal & air quality environment.

The regulator

At the base of the human brain is the hypothalamus. The hypothalamus functions like a car radiator valve. It makes decisions on blood flow rate and destination so as to ensure a good operating temperature for the body’s “engine”.
Such is human evolutionary history, that the hypothalamus has real power at the body’s decision-making table. It can divert up to 48% of the heart’s total blood flow away from critical organs to the skin (if you are too hot) or muscles (if you are too cool), so as to ensure the body is on it’s best survival footing. This extent of blood flow diversion deprives our liver, brain, lungs, stomach, kidneys, pancreas etc. When that happens, we don’t feel as well as we could, we don’t think as well as we can and we don’t heal as well as we can.

Figure 1 Temperature control is so important that critical organs will miss out on blood flow when the body is under thermal stress.

As one example, mortality rates in naturally ventilated spaces in an Australian aged care chain were reported in early 2001 as sevenfold that of air-conditioned spaces. This paper will help you understand why that type of statistic can come about.

The path of thermal comfort blood flow

There are two main blood flow paths through the body. In one path, the thermal comfort path, the heart pumps blood (now at a desirable temperature) through arteries, typically to a critical organ destination. There, the blood flows through tiny capillaries, effecting a tremendous exchange of heat, oxygen & nutrients, and then returns with by-products (such as carbon dioxide & heat) via veins to the skin level where it is either absorbing heat (because the blood is cold) or releasing heat (because the blood is hot).

When a body starts to over-heat, the first response is to accelerate heart rate but if that is not a fast enough response, some of the blood is diverted away from critical organs and goes straight to the skin to cool down.

Small shifts in body heat cause significant reactions within the body. The reader may recall when they visit a doctor, how quickly they measure your temperature. This is the body’s primary “tell”, that there is something wrong, often with temperatures as small as 0.80C above normal.

Figure 2 The hypothalamus takes quick action when there are small shifts in core temperature.

A shift in body core temperature as small as 1.50C can cause significant sweating or shivering. If that body core temperature is allowed to move to 50C away from normal, it would mean typically death. So the hypothalamus is tasked with great responsibility and acts powerfully to ensure the body survives.

The path of air quality blood flow

In the other blood flow path, the air quality path, arteries carry de-oxygenated blood to the lungs. There, the blood flows through capillaries releasing carbon dioxide and absorbing oxygen. Veins return the now-oxygenated blood to the heart’s mixing chambers where it can now be sent out to the critical organs.

Note that artery blood flow still carries oxygen, just at a reduced rate. It exchanges the gases of oxygen and carbon dioxide subject to the partial pressures (and therefore concentrations) of each gas in the lungs. This is why inadequate fresh air makes us tired. Firstly, our blood flow changes to compensate for reduced oxygen intake, and secondly the concentration of oxygen in our blood has dropped, meaning the burning of ATP fuel is reduced and we feel sluggish.

Other decision-makers

So far, I have simplified the description of blood flow as if it were a simple circulatory system with only 2 key functions. There are many other “decision-makers” in the body deciding what happens in this circulatory system, but let’s just acknowledge the next 2 most important decision-makers.

The nervous system (primarily the sympathetic system) manages the body’s reaction to stress, such as the fight or flight response. The sympathetic system readies the body for fight or flight and increases both heart rate and oxygenation in the blood, or rests it’s demands when the threat is removed.

Figure 3 Short-term over-ride by the nervous system modifies blood flow, direction, and chemicals.

The immune system also competes for blood flow. The blood’s purpose of carrying oxygen, nutrients, & white and red blood cells to sites of injury or soreness or to our lymphatic system is central to how well our body heals and feels after trauma. Again, this is a short-term over-ride.

These are the main systems that compete for- and dictate- heart rate, oxygenation, and blood flow. They compete so as to prioritise the ability to fight infection, heal injuries, provide energy, and filter toxins. In a healthy body, the hypothalamus generally has the most control over blood flow and destination, whilst the other two systems have more specific, short-term corrections that over-ride temperature control during a crisis.


Engineers will easily imagine the heart as the blood flow pump, and the brain (with its associated systems) as the blood flow controller. The actual “actuator” in the network for controlling blood flow is simply the diameter of the artery/capillary/vein structure. Muscles surround these blood vessels and they squeeze or open to divert or entice blood flow respectively. This is called vasodilation or vasoconstriction.

To an extent, the blood flow system is a self-balancing system, able to balance competing demands for blood flow and the nutrients it carries. It acts without conscious direction and, being the primary survival mechanism, it is very powerful.

There is a natural, physical limit though, to how flexible this circulation system can be, based in part on how supple the blood vessels are, cholesterol, alcohol, the amount of blood in the system, and other variables. For this reason, comfort standards have limits for temperatures so that our bodies have a chance of maintaining comfort in a moderate environment.

Cerebral blood flow

Blood flow to the brain comprises typically 15% to 20% of the heart’s blood flow, but it is worth mentioning because of how tightly controlled cerebral blood flow is. Low flow or poor oxygenation lends itself initially to brain cell death but ultimately could result in a stroke. High flow or pressure to the skull leads to headaches and ultimately to internal bleeding & tissue death.

The delivery of oxygen & nutrients to the brain (and removal of carbon dioxide from the brain) is prioritised by the body’s control systems over oxygen supply to other areas of the body. Small changes in the oxygen content we breathe can cause large blood flow variations in the brain. A 2% variation in oxygen content in the air we breathe can cause 5% variation in cerebral blood flow, which in turn can cause a 10% variation in cranium blood pressure. Coupled with other common problems, (such as dehydration from coffee-drinking), this change in blood flow can be the onset of a headache.

Figure 4 Effect on the heart rate of breathing the air where the oxygen concentration has diminished from 21% (outside air) down to 10% (poorly ventilated spaces).

The delivery of oxygen to the brain is fundamental for productivity. So much so, that the variable PMV is the only variable used in CHAM’s Productivity Loss formula. PLOS, the loss in performance in % by people occupying the space, is defined as:

PLOS = b0 + b1PMV + b2PMV2 + b3PMV3 + b4PMV4 +b5PMV5 + b6PMV6

where PMV is the Predicted Mean Vote and b1 – b6 are regression coefficients they publish.

Milton conducted a string of experiments on productivity using call centers. In plotting PMV versus productivity loss, there was found to be real productivity loss for higher PMV scores, as per the image below:

Figure 5 Milton’s experiments on PMV & Productivity show the diversion of blood in effect.

Significant changes in blood flow to various parts of the head stretch those areas and cause pain that we call a headache. Headaches can be caused by many external stimuli from the nervous system but the physiological impact is too much or too little blood flow to specific parts of the skull. One example my teenage son sets out to experience, is what he calls “brain freeze”, when he gulps significant ice-slushy in a rush. The brain is one of the fastest reacting organs for blood flow and acts quickly to stop heat loss through the head.

Figure 6 Our brains and shoulders have less body fat and are more effective at shedding heat.

It’s a myth that “we lose most of our body heat through our heads”. In truth, we lose body heat generally equally through whatever skin surface is uncovered. However, the head and shoulders are much more effective than other body areas for heat loss because we tend to have little body fat (insulation) in these areas. The origins of this myth are unknown, but the 2013 US Army Manual on Survival still maintains the assertion that we lose half of our body heat through our head.

Why should designers care?

Our bodies are so tuned for survival that they compensate for environment deficiencies “behind the scenes” in a manner that can be easily missed or dismissed by a casual observer. We rarely, as designers, appreciate the manner in which the environment can hinder or enhance body performance. The selection of outside air levels, temperatures, lighting levels and colour descriptions, acoustic environments, view connection to the external world, all impact our well being, our ability to heal and learn, and our performance at work.

In truth, our bodies can be easily impacted by relatively minor shifts in both the thermal and chemical environments. For example, consider the impact of a teaspoon of instant coffee on heart rate. Or consider the therapeutic effect of getting 15 minutes of “fresh air” at a break time. These seemingly innocuous events are signals that your body has been compensating (or will be compensating later) for a deficiency in the environment.

These environmental influences divert blood, raise blood pressure or affect our hydration levels and in turn affect how people heal, rest, concentrate, or feel.

What are the differences in age, gender, and ethnicity?

When driven by thermal necessity, differences in blood flow and pressure are proportional to metabolic rates. Metabolic rates alter regularly throughout the day in response to circadian rhythms, exercise, reproductive cycles, and diet, but generally, they decline with age, and males tend to have a 10% higher metabolic rate than females.

With respect to ethnicity, there is evidence that there are slight shifts in indigenous body core temperatures related to the climate, which we accord as acclimatisation. However, the real changes in comfort science for ethnicity are related to the body’s preferences for maintaining comfort. This is an essential difference that means designers can largely use most parts of existing comfort science without throwing everything out, and make adjustments for local ethnicities.

As an example, native Eskimos have higher fat ratios than Ango-Saxon Canadians. Similarly, some Indian races have a “vascular” age greater than their actual age, as their heart rate is consistently higher (from living in higher altitudes). There are further studies (somewhat ambiguous) that compare African Americans to their European counterparts and find hypertension and blood pressure is consistently higher. It is also well known that athletes who grew up in locations of high altitudes develop a higher lung capacity than the remainder of us.

In layman’s terms, our genetic code alters our body’s preferences for retaining or losing heat, and for absorbing oxygen, so as to survive with less effort.

We have a case study described on SEED TV where we review healing rates in a PNG hospital between air-conditioned wards and naturally ventilated wards. This TV episode presents one technique as to how we approach different ethnicities.

So what can designers do to improve the physiological well-being of occupants?

The guiding principles have been in modern design literature for a long time. These include:-

  1. Designing architectural fabric and HVAC with PMV & CO2 standards as a benchmark
  2. Designing with high levels of fresh air in the breathing zone. (The breathing zone is a hypothetical 1m x 1m x 1m cube around our head).
  3. Whilst these items may appear to be common sense, there are some challenges, such as what happens to air quality with heating systems where warm air stratifies, or cold windows with a view.

    Other principles that are perhaps less commonly considered include:

    1. Designing with heat loss mechanisms in mind. This might mean emphasising radiant or convective comfort for those who would respond well.
    2. Designing for individuality – that individuals have some control over their thermal and air quality environment.
    3. Designing for contextual differences on metabolism such as activity, age, ethnicity, season, or clothing requirements

    What must be stressed is that the approach required for a near-optimal solution is not the sole domain of a single discipline.

    Heat loss mechanisms

    It is often quoted that 43% of our body heat is lost through radiation from our skin. A further 35% of the heat loss is through convection (air movement with a temperature difference on our skin). A further 15% can be lost through sweating (and our glands are always sweating, in some sense).

    What must be remembered is that all of these statistics are derived from tests on mainly healthy Anglo-Saxon males. In truth, our body selects heat loss mechanisms based on the level of exercise, hydration, and available air movement. For example, a middle-aged male who wears a suit, who is dehydrated from coffee, and who rarely exercises, is much more likely to have a reliance upon convection (i.e. air conditioning) as a heat transfer mechanism than sweating. The inverse is also true (i.e. a young, fit hydrated female) – she is likely to prefer an environment with more oxygen such as a well-designed, naturally ventilated space.

    When looking for clues on heat loss mechanisms, I have always found it useful to study clothing habits. Large shady hats, removable tracksuits, tops with arms and shoulders exposed etc, are all clues to the designer about how people are making themselves comfortable, and by extension, how buildings they occupy can be made more comfortable.


    I have found this material useful in addressing and explaining comfort and air quality assessments for clients. In some simple projects, we have explained to school boards why a particular uniform policy may be detrimental to student comfort. In other projects, we have argued that diversity in control is more important than having a large central less-flexible control system.
    In the SEED TV video that accompanies this paper, we discuss a hospital in PNG where the Australian Government was funding a roll-out of air-conditioned wards. We spent time with the nursing staff to discover that patients who had moved into the air-conditioned wards had recorded higher blood pressure readings and slower healing rates. As we began to analyse the data, we realised that many of these patients were local indigenous populations who had never experienced air conditioning before. That story illuminates how critical blood flow is to effective body performance.