[Paper Reading] Investigation of indoor environment quality and factors affecting human comfort: A critical review

Publication year: 2019
Authors: Maedot S. Andargie, Marianne Touchie, William O’Brien

Introduction

Along with increasing urbanization, daily energy consumption is also increasing. The energy demand is expected to increase exponentially over time. Almost 70% of overall energy consumption in the world is used to maintain a favorable building indoor environment. Consequently, it is very important to maintain the balance between the energy requirement of the building and the indoor environment quality.
Several studies have established the relationship between thermal, visual, and acoustic comfort, IAQ and human health, efficiency, productivity, and occupant mental health. few standards are provided for the assessmentof IEQ in the International Organization for Standardization (ISO) 7730-2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 55–2013, and European Standards EN 15251-2007
Perceptions of comfort are different for different people under the same IEQ. Comfort is the combination of many factors so that even people who are in the same geographical area at the same time and the same age can also show conflicts in their perception of thermal comfort. The lack of inadequate indoor comfort can cause dissatisfaction in the occupants and negatively affect their productivity and performance, as well as various problems such as dryness, health, morality, and a well-known sick body syndrome (SBS).

Acoustic comfort

Acoustic comfort can be demarcated as “a state of satisfaction with acoustic conditions.” Many investigators have tried to establish a relationship between the effects of acoustic comfort on comfort and IEQ.
40 dBA background noise and 85 dBA white noise did not affect occupant comfort. Sound intensity can be measured in two ways, sound pressure level (SPL) over a long and short period and frequency. It found that the soundhad no effecton the occupants’ feeling of comfort for at least 30 min, but that when the frequency continued for more than 120 min, there was a considerable impact
The intensity and frequency of the sound can change the degree of discomfort of the occupant, as noise from people speaking nearby, doorbells, irregular and unexpected sounds create more discomfort for the occupant compared to regular noise of constant frequency

Visual comfort

Visual comfort is well known as “a subjective condition of visual well-being induced by the visual environment”
Visual comfort is vital for the productivity and well-being of the occupantsof indoor buildings, regardless of the type of activity they perform.
It is significant to examine the quality of lighting in the building envelope compared to the workplacewith artificial or natural lighting. Regardless of the type of natural or artificial lighting, an optimal balance between energy consumption and good lighting must be maintained to improve the productivity, comfort, and well-being of occupants

IAQ and thermal comfort

ASHRAE-55 outlines thermal comfort as “this mental condition that expresses satisfaction with the thermal environment and is evaluated by subjective evaluation”. Thermal comfort is the most important and commonly controlled parameter in the IEQ. Human beings are always seeking comfort in their occupied zone.
Physiological aspects include personal and environmental parameters, most studies focus on physiological parameters. There is a lack of detailed analysis about the occupant’s psychological state and their effect on the responseof the occupantto the IEQ
To judge psychological factors, extensive research has been conducted on empty rooms, busy offices, hospitals, office buildings, workshops, people who are awake, and performing certain tasks with the assistance of related questionnaires.
The term IAQ refers to drafts, blocked air, dry air, humid air, odor, and the concentration of pollutants in indoor air. The concentration of pollutants/dust particles influences the IAQ and the health of the occupants.

IEQ inference factors or thermal comfort indices

Thermal comfort is a sensation and is affected by the following physical factors: 1) air temperature, 2) ventilation, 3) air movement, atmospheric pressure (draft), 4) radiant heat, 5) humidity, 6) ambient temperature, 7) individual and combined metabolic rates 8) clothing insulation, 9) climatic conditions, 10) season, 11) age and gender 11) cleanliness
Taking into account the various environmental and personal factors, ASHRAE 55, ISO 7730, and EN 15251 propose the method of predicting the sensation of thermal comfort and the comfort of the indoor occupants.

Environmental factors

In an indoor environment, the main mutable factor controlled to provide thermal comfort is the indoor air temperature.
The main modes of heat transmission from the occupant to the atmosphere are conduction, convection, and radiation. 75% of the energy is dissipated by convection and radiation
The heat interaction by convection and radiation between the occupant and the indoor air is considerable. The degree of convection and heat interaction by radiation be governed by the airspeed, air temperature, skin temperature, and exposed surface area of the occupant.
Skin temperature is the temperature maintained by the outermost layer of human skin, commonly known as body temperature.
The comfortable temperature of human skin should be between 28 ◦C and 32 ◦C. For a low level of activity, this temperature is given at 34 ◦C

Air temperature

The air temperatureis the utmostsignificant factor to analyze indoor thermal comfort. The ambient air temperature surrounding the occu pant is generally known as the dry bulb temperature or air temperature. The overall temperature of the dry bulb must be between 18 ◦C and 23 ◦C for optimal thermal comfort conditions
Air temperature is the main variable that can be sensed by human skin to judge thermal comfort in the occupied zone. The acceptable indoor air temperature also depends on the type of ventilation used. In general a natural ventilated system, occupants can tolerate higher air temperatures between 25 ◦C and 28 ◦C
Neurobehavioral study shows that productivity and efficiency deteriorate when the air tem perature fluctuates from its average position. The occupant devotes more attention and energy to adjust to the ambient temperature.
The international standard stated that the air temperature should be between 20 ◦C and 24 ◦C, but the actual air temperature depends on the comfort requirements of the patient and operating personnel.

Radiation temperature

Numerous studies suggest that thermal comfort is primarily manipulated by the radiation temperature in the indoor environment compared to that of the outdoor environment, especially when the indoor environment is not mechanically controlled.
The mean radiative effect changes while moving farther or closer to the asymmetric radiation zone. The effect of radiation temperature in a space is much similar to that of the air temperature.
In ASHRAE, the assumption is made that the mean radiative temperatures are similar to the air temperature hence it is difficult to distinguish between radiation and air temperaturefrom the ASHRAE chart and may deludethe thermal comfort standards. Although there are methods for estimating the radiant temperature according to the guidelines of ISO 7726, there are no adequate and inexpensive methods to study the effect of radiant temperature on thermal comfort. Hence radiation temperature individually cannot define the thermal comfort of occupants.
To study the combined effect of air temperature and radiation temperature on thermal comfort, the concept of operative temperature is adopted in general for many studies. All the objects and surfaces which are at absolute temperature radiate some amount of heat and can be calculated using view factors and approximate temperatures, the average value of this radiating heat is termed as mean radiant temperature.

Operative temperature

The operating temperature varies throughout the indoor environment, although the indoor air temperature is unceasing, the radiation temperature is different at different positions in the room due to various factors such as the emissivity of electrical equipment, radiators, and machines surfaces.
It is the constant temperature of an imaginary black enclosure in which an occupant would exchange the equal quantity of heat by radiation and convection as in the real non-uniform environment

Comfort temperature

The comfort temperature reflects important constraints such as the radiation temperature, air temperature, and airspeed. If the airspeed is less than 0.1 m/s, the comfort temperature is reduced to the operating temperature since the operating temperature is the basic form of the comfort temperature.

Air movement

In large buildings, air movement depends on factors such as temperature sources, indoor and outdoor air pressure difference while in small buildings/offices/rooms, air movement depends mainly on air temperature.
In naturally ventilated buildings, thermal comfort has been found acceptable for air temperatures up to 28 ◦C by employing small fans and coolers using airspeeds between 0.5 and 2.0 m/s since air, velocity impacts convective heat loss from the human body and changes the thermal perception of naturally ventilated spaces
While the maximum airspeed mentioned in the ASHRAE 55 is 1.2 m/s in the condition where the paper is not blown due to the effect of the ceiling or table fan that can disturb occupants doing light activities
In 90% of the observed studies, it is found that the air velocity for the acceptable indoor environment is relatively low and rarely above 0.3 m/ s. For natural ventilation, the airspeed can be maintained above 0.3 m/s in hot and humid climatic conditions.
The key factors that influence draft are air velocity, air temperature, human metabolic rates, exposed body parts, coagulation factors, and fluctuating airspeed. The effect of drafts is major on the neck and ankle, which are small areas and an uncovered part of the body. The draft effect is maximum from the back as compared to the front at the same air velocity.
In general, the airspeed should be limited to 0.1 m/s for mechanical ventilation, especially for cold climates.

Relative humidity (RH)

It is generally given in percentage and defined as the ratio between the water vapor present in the air and the maximum possible amount of water the vapor contained in the air at a known temperature.
In a naturally ventilated indoor environment, humidity is highly reliant on the season and time of day and this can be controlled manually by an occupant using various comfort equipment
In general, it is found that humans are not able to sense humidity, humans do not have specific receptors to sense humidity but indirectly can be felt using other causes such as temperature receptors [94]. The perception of dry air (low humidity) is possible due to irritation of the mucous membranes of the nose and dry eyes, which is the main parameter used in the study of SBS
According to the standards given by standards ISO 17772, EN 16798, and EN 15251, the RH must be between 30 and 50% for people very sensitive to humidity such as women, and between 25 and 60% for normal occupants.
The recommended relative humidity for the in door hospital environment is 30%–60%, while higher relative humidity values increase the risk of bacterial growth and thermal discomfort.
In general, the relative humidity factor is less important in the analysisof thermal comfort, and many studies have maintained the RH value in the range of 40 –50% to control sensory irritation and other indoor pollutants

Contamination and air change rate (ACR)

Climate conditions

Personal factors

Metabolic rate

Clothing insulation

Thermal history

Prediction of thermal comfort

Thermal comfort evaluation

Thermal comfort models

Overall discussion and limitations

Conclusions and trendsetting

This study focuses on the last 50 yearsof IEQ and thermal comfort. The main focus was kept on recent 20-year papers, while some older novel and notable works are also considered for discussion. Designing the dormant building is no longer enough nowadays; it must be tested for the health, performance, and well-being of the occupants according to the building design.
Indoor occupants tolerate low-level noise and gain a state of satisfaction with acoustic conditions.
An individual state of visual comfort prompted by the visual environment essential to human well-being and productivity. In addition to artificial lighting, daylight glazing advances visual comfort and reduces the energy consumption of the building.
Improving indoor thermal comfort depends mainly on two major factors, air temperature, and air movement. The calculation of parameters such as operating temperature and comfort temperature useful in determining the overall effect of air temperature on IEQ.
The relative humidity of the indoorclimate is fixed at 50 –60% for all types of climatic conditions, regardless of the geographical conditions.
To improve IAQ and minimize the threat of cross-infection from various airborne diseases, the higher ACR at low airspeed should be used reliant on the occupancy capacity of the room. But a higher ACR adversely affects the building energy consumption.
Most studies focus on the fixed value of metabolic rate as a function of activity level and thermal history, but the metabolic rate varies continuously with time and ambient conditions. The increased metabolic rate reduces the temperature range of thermal comfort and humidity.
Clothing insulation can change the human perception of thermal comfort. The clothing insulation varies according to the geographic, climatic, and thermophysical properties of the occupant. The thermophysical and insulating properties of the fabricof the garmentmust be studied to obtain thermal comfort for low energy requirements.
Very few models specify the depth and assess all the aspects, namely thermal, acoustic, and visual comfort. Individual and socio-economic factors are not taken into account. Well-known models also ignored the effect of visual and acoustic comfort. There is a need for a more personalized model for IEQ study and thermal comfort.