Thermal sensation

The thermal sensation is called the reaction of the human body to the set of conditions that determine the inhabited environment from a thermal point of view. It is customary to say that it is hot or cold, depending on what an ordinary thermometer says, but not only the temperature that the thermometer gives determines the sensation that the human body feels, but, at the same temperature of the thermometer, there are other parameters that influence, being able to improve or worsen the sensation.

Parameters to evaluate sensation

Studies have been done to reflect these sensations in the form of indices that take into account two or more of the parameters that affect the sensation.

These parameters can be, either of the environment in which the human body is considered, and they are:

• Dry temperature
• Average radiant temperature (or black bulb temperature).
• Relative air humidity, which can be calculated from the wet temperature.
• Air speed.

Good of the conditions that affect the person:

• Metabolic index, the heat produced by the body.
• Indumento index, the greater or less insulation produced by the indumentary that is worn, indumentary that is the set of clothes, footwear and touched.

Exchange mechanisms

The human body uses energy that it gets from food. It uses it in growth, in movement and in its own functioning. As waste, it produces heat, which is first used to keep the body at the proper temperature for operation and then dissipated into the environment. The amount of heat produced is evaluated by the metabolic rate.

If more heat is dissipated than produced, you feel cold; if it dissipates less, there will be a sensation of heat. The sensation of comfort is obtained when the heat dissipated is approximately equal to the heat produced. Any mechanism that alters heat loss from the body will influence the thermal sensation.

The naked body has the possibility of regulating the emission of heat for ambient temperatures between (approximately) between 15 and 30 °C. Above and below it has to do something.^{[citation needed]}

In this sense, the thermal sensation is conditioned by the environmental conditions that facilitate or prevent the proper dissipation of excess heat.

Body-environment heat exchange

The human body exchanges heat with the environment in four ways:

• Driving: when the naked skin, or with clothes of little coat, comes into contact with an object at different temperature than the skin, exchanges heat with the object by driving; it is the least significant means of exchange, because it is usually go dressed and shoes.
• Convection: the air in contact with the skin warms and increases volume, therefore loses density, ascends and is replaced by cooler air. If the air is hotter than the skin, the process will be the reverse. It occurs in naked skin parts or covered with very loose clothing.
• RadiationEvery body exchanges heat by radiation with another at different temperature, without contact with it. Evidently in the human body this exchange is made by naked or unbridled parts.
• Evapotranspiration: the evaporation of the water in the skin requires a quantity of heat of 2450 kilos per kilogram of evaporated water (540 kcal/kg), heat obtained from the body. The magnitude of this exchange depends on relative humidity and air speed.

The sum of the amounts of heat exchanged by each of these phenomena is equal to the heat lost by the human body and, for comfort, it must be equal to that produced. It must be taken into account that in the first three processes, heat exchange depends on the temperature difference between the skin and the environment, the thermal jump. The greater the thermal jump, the greater the exchange. For ambient temperatures below those of the body, the balance is negative (the body loses heat) which is what it needs to do, but the balance can also be positive, that is, if the ambient temperature is higher than that of the human body, the body will gain heat, when it needs to lose it, always. The fourth system, evapotranspiration, always involves a loss of heat for the body, so that if the ambient temperature exceeds that of the body (temperature above 36.5 °C) the only way to dissipate heat to the environment is through the sweating.

There are a number of fixes to this schema. In the first place, the processes occur mainly on bare or poorly protected parts of the skin; on the other hand, the temperature of the skin is not equal to that of the interior of the body, but normally lower. In addition, the body varies this surface temperature depending on the needs; Thus, in cold weather, it reduces superficial blood flow, lowering the skin temperature, to reduce the thermal gap and reduce losses. The reduction in blood supply can be significant, to the point that the superficial skin can become necrotic (chilblains) due to lack of irrigation and the extremities can become frozen in extreme cases. On the contrary, in case of significant heat, the superficial irrigation increases to increase the jump and the dissipation of heat, although it can never rise above body temperature; In any case, when the ambient temperature is higher than that of the body, increasing the surface temperature reduces the thermal jump, and decreases heat absorption.

Modification of the conditions

Depending on the ambient temperatures, you can act:

• In cold weather: the production of heat can be increased by increasing body exercise (metabolic index) or, the loss of heat can be reduced by adequate thermal insulation, which is achieved with the indumentary, adding to the clothing the footwear and the tocquer (indumento index).
• In warm weather, where it is more difficult to lose heat, production can be reduced by immobility or loss can be increased without as much clothing as possible (thermal insulation). Contrary to what many think, it is not a solution to go almost naked on the outside, since the skin is an excellent absorber of radiation and the sun a very powerful emitter, so you have to protect yourself from that radiation. It is very appropriate to use the lightest color clothing possible (the white is the color that less absorbs the radiation) and that the clothing is loosed to allow a better convection and consequent evaporation of the sweat. Desert men (tuaregs and similar) are completely covered to avoid receiving solar radiation.

A means of increasing the losses in warm time is to provoke a movement of the air around the subject, either by means of a fan, or by using the different openings of the premises (windows, doors) to produce an air current. The air speed facilitates the evaporation of the sweat (renewing the wet air glued to the skin, by dryer air), and increases the heat exchange by convection, so the heat losses are greater by increasing the air speed.

In the sensation of heat or cold there are a series of parameters that influence and among them can be counted:

• From one's own human body:

In this aspect, it is necessary to consider that not all people are equal in the sense of heat or cold; they are usually less frioleros fat than thin ones, women than men, young people than old ones, although, obviously, this is full of exceptions.

More generally, in the case of heat production, the amount is increased mainly by the exercise that is done (metabolical index); for example, on a cold day, 25 people run through a football field in t-shirt and short trousers (and also sweat copiously), while in the stands are stacked 20 000 viewers warm and having cold.

You don't have to despise the food: in cold times you eat more quantity and you prefer hot food (puchers, stews), while in warm times you eat less and more cold dishes (assaults, tomato gazpacho, for example), that absorb heat from the body to reach its temperature, as well as more vegetables (they have less amount of "energy" food).

• Environmental:

The thermal sensation can be of greater temperature when the heat is added to a relative high humidity since, in this case, the evaporation of the sweat is the main means to dispel the body heat and, the high environmental humidity makes this evaporation difficult, so it has a feeling of more heat.

In the premises, radiation from walls at greater temperature than the environment can cause, having a relatively low air temperature, the feeling that it is hotter. On the contrary, cold walls will give a colder feel, even if the air of the place has a proper dry temperature.

• Aclimate:

An important issue is that produced by abrupt temperature changes. In this regard there are two situations to consider: the change of climate after a trip or the change of climate after entering a climate-conditioned place from the outside with an extreme situation of cold or heat.

Travel: This acclimatization began to be studied when, in the Second World War, English troops were transported by plane to the Pacific from England; a very low performance was found in the troops entering combat almost immediately upon arrival, and they realized that they required a acclimatization to the heat of the tropics before doing so, to be able to withstand temperatures they were not used to. Previously, with boat trips of many days, acclimatization was produced naturally, gradually.

Local: when you enter a heated place from a cold exterior there is the possibility of changing clothes from clothing, which reduces the feeling of heat. However, when the opposite end is given, moving from a very warm exterior to a refrigerated place, it does not usually take shelter clothes and a cold blow can occur with a catarrh result or other related diseases. It is therefore recommended that cooling temperatures be as high as possible to avoid it; about 25 or 26 °C is a fairly adequate temperature (and also very convenient for energy savings).

Thermal Comfort Indexes

To define the thermal sensation, taking into account the previous parameters, a series of indices are used, most of which take the name temperature, and are Its unit is the Celsius degree, to assimilate it to a parameter that everyone knows, a value that is normally associated with the thermal sensation.

To explain it: the index would reflect that the sensation of a subject in the studied environment would be the same as that in a closed space, in which the air and the walls (walls, floor and ceiling) were at a temperature equal to the index, with the air at rest and the relative humidity equal to 100%. From this definition of the index, it can be deduced that in an environment such as the one described, the exchange by convection, conduction and radiation takes place at the temperature of the index, while that the air at rest and the relative humidity of 100% completely prevent the exchange by evapotranspiration or sweating.

When these indices have a value between 18 and 27, we can speak of thermal comfort for a person at rest and with normal clothing (clothing index between 0.7...1 clo). It goes without saying that these values are considered to serve a significant percentage of potential users, but there are always some whose environmental requirements are outside this range. These indices are normally used for enclosed spaces, although they can also be used to describe an outdoor environment.

• Equivalent temperature or Operating temperature: take into account the dry temperature and the medium radiant temperature.
• Outcome temperature: besides the dry temperature and the radiant temperature takes into account the air speed.
• Effective temperature or New Effective Temperature it takes into account the dry temperature, the relative humidity of the air (or the wet temperature) and the speed of the wind.

By the definition given above, the indices are expressed in celsius, although their formulas include different units. It is a mode of expression that everyone can understand, compared to the dry temperature.

Representation of PMV on a psychmetric diagram

There is an index that is not expressed in degrees, the estimated average opinion or predictable average opinion, which takes into account that not all people react in the same way to certain characteristics of the thermal environment; there have always been people who are more chilly and others less wholly, so to take this issue into account, it is defined:

• Estimated average.— Call, in Spanish law, Average esteemed votetranslated almost literally from English Predicted Mean Vote and is known by the acronyms PMV (of English expression), and that a more approximate translation would be Predictable discharge. It is due to Povl Ole Fanger the establishment of this index, which reflects the average statistical opinion of a large group of people regarding the thermal sensation perceived in a given environment, defined by a set of six parameters. The index covers a range of seven values, between -3 (cold) up to +3 (heat).

Indeed, it is almost impossible for a given combination of parameters values to be able to satisfy everyone from the thermal point of view. For this index it is considered that the opinion is closer when the value is approximate to zero, but it is considered appropriate values that include -0,5 metrics.

Partial indexes

There are other partial indices for extreme cases of heat or cold, generally outdoors, especially in the second case:

• Bochorno temperature, in Canada the term is used Humidex indexAnd Americans call it Heat index English heat index) that takes into account the interaction of high dry temperatures and relative humidity. It is an incomplete index, since it does not contemplate the air speed, a question that can alleviate the bochorno in an important way.
• Temperature of sensation(in English) wind chillwhich would be translated by cooling by the wind) temperature seated or, more appropriately, perceived temperature index: takes into account the interaction of low or very low temperatures with the speed of the wind. As the previous one, it is partial, since it does not contemplate the influx of radiation, which outdoors can be noticeable on a sunny day.