The purpose of this paper is to review the existing literature to draw an understanding of the relationship between indoor environmental quality and occupant productivity in an office environment. The study reviews over 300 papers from 67 journals, conference articles and books focusing on indoor environment, occupant comfort, productivity and green buildings. It limits its focus to the physical aspects of an office environment. The literature outlines eight Indoor Environmental Quality (IEQ) factors that influence occupant productivity in an office environment. It also discusses different physical parameters under each of the IEQ factors. It proposes a conceptual model of different factors affecting occupant productivity. The study also presents a review of the data collection methods utilised by the research studies that aim to investigate the relationship between IEQ and occupant productivity. The study presents a comprehensive discussion and analysis of different IEQ factors that affect occupant productivity. The paper provides a concise starting point for future researchers interested in the area of indoor environmental quality. Keywords: occupant productivity, workplace satisfaction, indoor environment quality, occupant comfort Paper type: Literature review Humans spend most of their time indoors, and the majority of the world's population lives in urban areas and work in an office environment (ASHRAE, 1993). There has been a significant global shift in the economy from the manufacturing sector towards the service and knowledge-based sectors which operate in indoor office environments (Haynes, 2008b , World Green Building Council, 2014 . Hence, it is becoming important to understand the indoor office environment and the effect it has on occupant productivity. An office environment has a high level of influence on its occupants' productivity (Leaman and Bordass, 1999 , Frontczak et al., 2012 , Roelofsen, 2002 , Mawson, 2002 , Van der Voordt, 2004 . Past studies on sustainable buildings postulate that green design strategies and technologies enhance the indoor workplace environment. Such strategies/technologies enable the creation of an environment which favours occupants' comfort and performance in both newly built and retrofitted buildings (Romm and Browning, 1994) . The majority of the building stock that will exist in 2050 has already been built (2009) . Thus, there is a need to investigate the quality of the indoor workplace environment and its relationship to occupant productivity. This paper establishes the state-of-the-art on environmental factors that influence occupant productivity in the office environment. It also highlights and discusses various occupant productivity measurement methods used in indoor environment research studies. This research will help construction industry professionals improve the designs that allow better operation of office buildings along with improving the productivity of the occupants. The research on the direct and indirect effects of indoor environment quality on its occupants' productivity can be traced back to 1920s when Maslow, as well as Vernon & Bedford published their work on the workplace environment and needs (Maslow, 1943 , Vernon and Bedford, 1926 , Vernon and Bedford, 1930 . Herzberg (1966) and Heschong (1979) were early researchers who highlighted the influence of the indoor physical environment on its occupants' productivity in a workplace environment. Later, researchers stressed on the degree of influence of the built environment on its occupants' productivity. Research studies have provided evidence on the impact of the indoor environment on an organisation's operational expenses (Romm and Browning, 1994 , Leaman and Bordass, 1999 , Oseland, 1999 , Heerwagen, 2000 , Brill et al., 1985 , Marans and Yan, 1989 . The operational cost of an organisation is divided into personnel costs, material costs, financial costs and building related cost (Feige et al., 2013) . In a study, it was seen that 25 years of companies' operational costs include around an 85 percent share of personnel costs (CABE, 2005). Other studies report that employees' annual salaries exceed the building operation and rental costs by a factor of up to 25 (Clements-Croome, 2000 , Fisk, 2000a . A marginal change in occupants' performance can produce a massive impact on a company's operational costs. Research focusing on the USA estimated that improved work efficiency nationally could yield up to US$12-125 billion yearly (1996 US$ value) (Fisk, 2000a) . A nationwide survey-based study using professionals from different sectors in the UK indicated that a good office environment can help to increase productivity by up to 20%, equivalent to £135 bn per year , Clements-Croome, 2015 . The UK Centre for Mental Health coined the term 'presenteeism'. It refers to employees who are present at work but work with reduced productivity. Presenteeism plays a significant part in the reduced productivity within the built environment sector. It leads to loss of £15 billion per year in the UK (2011). The indoor environment influences the health and well-being of the occupants. Healthy buildings lead to more flourishing and happy inhabitants (Clements-Croome, 2013 , Mendell et al., 2002 . It is the moral responsibility of the built environment industry to deliver a built environment that is conducive to its occupants and promotes their health and well-being rather than providing buildings merely fit for survival. Efficient and conducive workplaces help to reduce employee absenteeism, reduce staff turnover, and increase occupant productivity and satisfaction, thus increasing the perceived health and well-being of their occupants (US Green Building Council, 2004 , McGraw-Hill Construction, 2014 . All this evidence highlights the impact of indoor environment quality on employee productivity and emphasises the importance of understanding Indoor Environment Quality (IEQ) and its effects on occupants' productivity. The rest of this paper is divided into five sections. The first section describes the methodology of the study. It is followed by a section on literature review on occupant productivity. It reviews the relationship between green buildings and occupant productivity. An elaborate investigation of the scientific studies on occupant comfort and productivity in the built environment leads to the eight Indoor Environment Quality (IEQ) factors that affect occupant comfort and productivity. It reviews the relationship between each IEQ factor and occupant productivity. The third section provides an overview of the data collection methods used in the studies investigated in the literature review. It describes different methods of data collection that can be used for measuring the impact of Indoor Environment Quality on occupant productivity. A discussion section follows to discuss all the IEQ factors. Each factor is discussed individually along with the associated measurement technique that can be used to define the relationship between occupant productivity and the respective IEQ factor. The last section presents the conclusion of the study. The study was developed using a wide-ranging literature review looking at journal articles, conference articles and books to establish a firm base for the research findings. The study followed a four step process to identify, collect and classify the literature 1. In step 1, the keywords for the research were identified. The aim of the research was to determine the impact of Indoor Environment Quality on occupant productivity in an office environment. The keywords for the search were: occupant productivity, workplace satisfaction, indoor environment quality, occupant comfort, thermal comfort, indoor air quality, office layout, and occupant satisfaction. 2. The primary emphasis of the research was to identify and collect evidence on different IEQ factors' impact on occupant productivity. The authors used the University library's online search engine and Google Scholar, Science Direct and Elsevier for the literature search using the keywords defined in step 1. 3. After reviewing the initial group of papers, the research team looked at the bibliographies of the downloaded papers to look for more relevant papers. 4. The final step involved a classification of the literature based on the following two aspects: a) Year of Publication (1926Publication ( -2015 Articles were analysed according to the year of publication to draw an overall picture of the topic's research and development in the field of the built environment. The literature reviewed ranged from the 1920s to 2015. 1920 -1979 1980 -1989 1990-1999 2000 -2009 2010 -2016 Literature review -Year of Publication Following these steps the review of the literature was developed which is presented in the following sections. Several researchers have discussed the relationship between the performance of buildings and their occupants' productivity (Heerwagen, 2000 , Romm and Browning, 1994 , ClementsCroome, 2015 , US Green Building Council, 2004 , Miller et al., 2009 (Singh et al., 2010 , Romm and Browning, 1994 , Heerwagen, 2000 . A study of 500 LEED rated green buildings proved that a high-performance building or 'healthy' building helps to reduce absenteeism, increases occupant productivity and improves employee well-being (Miller et al., 2009) . Green buildings decrease operational costs by reducing energy costs and increasing occupant productivity (Gabay et al., 2014 , Clements-Croome, 2004b . Green buildings have a higher built asset value as compared to non-green buildings and they contribute to spreading awareness concerning the environment and efficient building operations (Roberts and Newell, 2009 , Clements-Croome, 2004a , Clements-Croome, 2004b , Clements-Croome, 2015 . Overall, there is a sense of awareness about the significance of IEQ's impact on occupant productivity. However, there is a lack of direct focus in green building guidelines to address this aspect and increase green buildings' performance in occupant productivity dimension (Potbhare et al., 2009) . LEED green building rating system has indoor environment quality category that focuses on thermal comfort, indoor air quality, lighting and daylighting, quality of views, lighting and daylighting and acoustic performance. These criteria affect occupant comfort and productivity but these are mainly related to the mechanical aspects of indoor environment quality assessment. The main categories in LEED rating system are also set up focussing on the engineering aspect of the buildings such as site usage, energy, water conservation, building material components, and recycling of waste materials (Lee and Guerin, 2009 ). There is no direct effort to cater occupant satisfaction and promote occupant productivity. Similarly, BREEAM has health and wellbeing category outlining issues such as visual comfort, indoor air quality, thermal comfort, acoustic performance, private space, accessibility, hazards, water quality, and safe contaminants in laboratories (Lee, 2013) . Majority of these issues/factors contribute in occupant productivity. However, BREAAM doesn't explicitly relate these issues to occupant productivity. Most of the occupant health and comfort parameters could result in higher occupant productivity. However, this might not be the case in every situation. Research states that it is not necessary that optimum thermal comfort and optimum productivity coincides within acceptable thermal comfort range (Fisk, 2000b) . There is an opportunity to analyse indoor environment quality criteria in various international green building guidelines. These criteria can be updated to focus on occupant health and productivity rather than just focusing on building performance. Productivity is defined as the ratio of output to input (Oseland and Bartlett, 1999) . The definition of productivity can vary, depending on the context and content of the input and output. In organisational operational terms, productivity can be described as the ratio of company turnover to employee cost (Oseland and Bartlett, 1999) . In the case of office environments, performance/productivity can be measured using different criteria such as individual performance, team performance and organisational performance (Heerwagen, 2000 , Feige et al., 2013 , Oseland, 1999 . In case of built environment, the input is the operational cost of a building and output is the building's operational performance. The output of an office building should include both building and occupant performance to give a more comprehensive picture. This paper explores indoor environment quality factors that have impact on productivity. It also highlights the nature and significance of the impact of these IEQ factors. Four main aspects that affect human productivity in an organisational working environment are personal, social, organisational and environmental factors. These factors influence the overall productivity of the employees. The level of influence and the inter-relationships of these individual factors is still unclear . A favourable environmental setting for occupants reduces the number of complaints and absenteeism and enhances productivity (Clements-Croome, 2000 , Lorsch and Abdou, 1994 , Bordass et al., 1993 , Tse and So, 2007 , Zhai et al., 2013 ). An office environment is considered as a healthy indoor environment when 80 percent of its occupants are satisfied with the environmental settings (ASHRAE Standard, 2004) . Occupants' satisfaction with the indoor environment is directly related to the amount of comfort they experience within the environment (Frontczak et al., 2012 , Choi et al., 2009 , Brager and Baker, 2009 , Heerwagen, 2000 . Employees' comfort with the environment has a direct impact on their workplace satisfaction which highly influence their productivity. Comfort is defined by an absence of unpleasant sensations, thus providing positive effects on well-being. It is subjective in nature and varies from person to person. There are different types of comfort in an office environment. It includes physical comfort (air quality, climate, noise), functional comfort (disturbances, interruptions, distance from work, resources) and psychological comfort (privacy, territoriality) (Feige et al., 2013) . Comfort in an office environment can also be affected by physiological components, psychological components and physical environmental conditions (Oseland, 1999) . Oseland suggests that physical environmental conditions can be broken down into physical conditions (temperature, light, noise, air quality), space (plan, layout and privacy), ergonomics (workstation and controls), and aesthetics (colour and quality). Overall comfort is an outcome of personal health and mood, in addition to functional as well as environmental factors (Leaman and Bordass, 1999, Clements-Croome, 2006). Researchers suggest that the behavioural environment, which comprises different behaviour and social dimensions in a workplace, affects overall occupant comfort (Haynes, 2007a , Fleming, 2004 . Behavioural and social aspects such as privacy, collaboration, interaction and distraction are subjective and influence occupants ' comfort (Brenner and Cornell, 1994 , Fleming, 2004 , Heerwagen et al., 2004 , Ward and Holtham, 2000 , Haynes, 2007a , Haynes, 2007b . Haynes emphasises that the behavioural environment is an integrated dimension of an office environment, and it affects occupants' behaviour and the social environments which are created and evolved by office workers. Haynes (2007a) proposed a theoretical framework, suggesting that office occupiers and work patterns influence the physical and behavioural components constituting the office environment, and both have a collective impact on office productivity. He concludes that office layout and comfort are part of the physical environment, while distraction and interaction are part of the behavioural environment (Haynes, 2007a , Haynes, 2007b , Haynes, 2008a . The definition of comfort in the literature has a broad theoretical spectrum ranging from physical comfort and personal health to social psychology. This paper limits the focus to the physical comfort that directly relates to the Indoor Environmental Quality (IEQ) of a workplace. The study identifies eight physical factorswhich affect occupant satisfaction and productivity in an office environment based on the findings in the previous literature:  Indoor Air Quality and Ventilation (Vernon and Bedford, 1926 , Fanger, 1988 , Fisk et al., 2012 )  Thermal Comfort (Fanger, 1970 , De Dear et al., 1997 , Tanabe et al., 2007 , Djongyang et al., 2010 , Lan et al., 2011 )  Lighting and Daylighting (Hopkinson et al., 1966 , Alrubaih et al., 2013 , L Edwards, 2000 , Sivaji et al., 2013 )  Noise and Acoustics (Sundstrom et al., 1994, Banbury and Berry, 2005, Mui and Wong, 2006)  Office Layout (Brill et al., 1985 , Laing et al., 1998 , CABE, 2005 , Haynes, 2009 )  Biophilia and Views (Heerwagen and Orians, 1984 , Grinde and Patil, 2009 , Heerwagen, 2009 , Bright, 2012 )  Look and Feel (Mahnke, 1996 , Kwallek et al., 1988 , Ou et al., 2004 Building Council, 2014)  Location and Amenities (Duffy et al., 1992 , Gordon-Larsen et al., 2009 , World Green Building Council, 2014 These eight factors have significant interactions and crossover between them. Daylighting has direct interaction with thermal state of an office. Windows absorb and transfer significant amount of solar radiation into indoor environment (Lyons et al., 2000) . A change in thermal state may have an impact on occupant perception of indoor air quality. A decrease in temperature leads to improved occupant perception of indoor air quality (De Dear and Brager, 2002) . Similarly, there is a crossover between daylighting and 'look' and outside views of an indoor environment. Occupant comfort and 'feel' of an indoor environment may get affected by the quality of outdoor views and influence the impression of an office environment (Aries et al., 2010 , Laet et al., 2002 . Also, the layout of an office can have an impact on its acoustic properties (Lee, 2010, Kim and. The study acknowledges the crossover and interactions amongst these IEQ factors. However, the study divides them into eight categories to collate the evidence of their individual impact on occupant productivity to underline their relevance in indoor environment design of an office. This review can be taken as starting point to develop experiments to look at magnitude and consequences of various interactions between these indoor environment qualities. Although literature discussed above factors focuses on indoor environment quality, there is a lack of focus on occupant productivity. In light of the impact, above factors should be reviewed or their impact on occupant productivity. The next section presents a state of the art literature review in the above eight IEQ factors. Physical Environment Factors affecting Indoor Environment Quality Indoor air quality (IAQ) is the degree of quality of the indoor air of a building. IAQ has a high impact on office productivity. A workplace with good air quality has higher work performance in office tasks such as text typing, proof-reading and mathematical tasks (Wargocki, 2000 , Langer and Bekö, 2013 , Ng et al., 2012 , Fanger, 2000 . IAQ studies on existing buildings following industry air quality standards have found many participants within existing buildings to have a substantial dissatisfaction rate with IAQ . They highlight various health related issues reported by some of the building occupants (Fisk et al., 1993 , Mendell, 1993 , Bluyssen et al., 1996 . These issues include SBS (Sick Building Syndrome) symptoms, allergies and asthma symptoms (Fisk and Rosenfeld, 1997) . SBS is a building related illness. The most common SBS symptoms are dry, itchy, sore and burning eyes alongside an irritated nose and sinusitis symptoms. Other symptoms include respiratory irritation, headaches, lethargy and mental fatigue (Hodgson, 2000 , World Health, 1982 . Global observation of these symptoms and illnesses has led to research efforts towards understanding indoor air quality and its components. Indoor air quality is a complex entity to measure. Various time-dependent physical and chemical parameters (such as relative humidity, temperature, level of air contaminants) are constituents of IAQ. These parameters are affected by outdoor conditions (climate), building conditions (material, structure and construction), buildings' HVAC systems (Heating, Ventilation and Air-conditioning systems), the indoor space arrangements (furnishing, furniture, equipment) as well as occupants' productivity patterns. These factors are highly interdependent and are dynamically affected by multiple variations and interactions (Szczurek et al., 2015) . Indoor air quality can be managed either by increasing the ventilation rate or by reducing the air pollutant load in the air. Ventilation is used to exchange the indoor air and remove carbon dioxide and other contaminants in the air (Kosonen and Tan, 2004, Seppänen et al., 1999). The ventilation rate is an efficient monitor of IAQ in a building. Higher ventilation rates result in good indoor air quality. Lower indoor air quality is associated with Sick Building Syndrome (SBS) symptoms and reduced productivity (Ezzeldin and Rees, 2013 , Kosonen and Tan, 2004 , Seppänen et al., 1999 . Research on european countries highlight that ventilation rates below 10 Ls -1 (litre per second) are associated with higher risk and prevalence of Sick-Building Syndrome and other health risks (Dimitroulopoulou and Bartzis, 2013 , Seppänen et al., 1999 , Fisk et al., 2009 ). There is a debate in the built environment sector that higher ventilation rates result in higher energy consumption. Researchers argue that higher occupant productivity by better IAQ would lead to an overall benefit for both the occupants and the building. Higher ventilation rates in a building could result in a financial return from ten to sixty times greater than annual energy and maintenance costs (Kosonen and Tan, 2004 , Wargocki et al., 2003 , Wargocki et al., 2000 . A research study on the quantitative benefits of an increased air ventilation rate in offices across the U.S. supports the argument. The study reports the yearly economic benefit of US$13 billion by increasing minimum ventilation rates from 8 to 10 l/s per person and US$38 billion by increasing ventilation rates from 8 to 15 l/s per person on a U.S wide scale (Fisk et al., 2012) . These benefits reflect an appreciation of better occupant performance and productivity due to better air quality. There are different ventilation systems available to control the ventilation rate of buildings. These mainly include naturally ventilated systems, hybrid/mix mode systems and mechanically ventilated systems. Some studies indicate that mixed mode HVAC systems have higher air quality satisfaction and energy savings than other HVAC systems (Ezzeldin and Rees, 2013, Gou et al., 2014). Literature also suggests that relative to natural ventilation, air conditioning, with or without humidification had significantly higher association with one or more SBS symptoms (30 -200%) (Seppänen and Fisk, 2002) . However, a ventilation system should be chosen based on various local climate factors, building type and occupant behaviour pattern and expectations (Kim and de Dear, 2012) . Mechanical ventilation system is extensively used in buildings in hot climate. However, Personalised Ventilation (PV) system can be highly effective for offices in hot and humid locations such as Singapore. PV system used as a secondary system in conjunction with primary air conditioning system can reduce overall energy consumption by 15-30%. It supplies fresh air directly to occupant's breathing zone. The combined effect of PV and main air-conditioning helps to enhance IAQ acceptability by the occupants (Sekhar et al., 2005) . A decrease in work performance can relate to an increase in pollutant load in indoor air quality. Formaldehyde and Volatile Organic Compounds(VOCs) are the main pollutants in indoor environment (Kim et al., 2001 ). Chemical and Microbiological Volatile Organic Compounds (VOCs/MVOCs) in the air have a direct impact on indoor air quality (Wolkoff, 2013a) . VOCs can be man-made or of natural origin. Natural origins of VOCs are oceans, volcanos and forests. Human activities in industry, petro chemistry and vehicular emissions are one of many reasons for VOC emissions (Panagiotaras et al., 2014) . Apart from indoor human activities like smoking and cooking (fume), indoor elements of a building such as carpet, furniture (wood preservatives), paint products, and cleansers are also responsible for VOCs emissions (Panagiotaras et al., 2014 , Chao and Chan, 2001 , Berenjian et al., 2012 , Association, 2011 , Guenther et al., 1995 . New buildings have higher VOCs levels than old buildings. Studies suggest that over a period of time, majority of buildings exhibit decreasing tendencies of VOCs in indoor environment (Shin and Jo, 2013, Langer and Bekö, 2013) . The majority of VOCs exhibit irritant and odorant properties (Wolkoff, 2013b , Jones, 1999 , Panagiotaras et al., 2013 . There are industry standards and guidelines recommending acceptable levels of contaminants (World Health Organization, 2005 , ASHRAE Standard, 1989 . However, VOCs exhibit a broad diversity in their physio-chemical properties. This makes it difficult to develop standard measures for VOCs for sampling and analysis. Sampling methods depend on compounds of interest and, in particular, on specificities and sensitivities relating to human physiology (Panagiotaras et al., 2013 , Teichman et al., 2015 . This subjective reaction to diverse compounds makes observing and mapping VOCs in a particular environment quite complex task. The most commonly used methods for indoor air sampling and analyses are TO (Toxic Organic)methods (U.S. -EPA Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air) and IP methods (Compendium of Methods for the Determination of Air Pollutants in Indoor Air) (US-EPA, 1999 , Riggin, 1984 , Panagiotaras et al., 2014 , US-EPA, 1990 ).Fanger (1988 introduced Olf and Decipol units to measure indoor contaminants and their sources (such as buildingfurnishing material and ventilation systems). Olf is the emission rate of air pollution. It is equivalent to the emission rate of the air pollutant from one standard person. Building materials' emissions have been estimated at 0.1 -0.2 olf/m 2 . Decipol is used to represent the level of perceived air quality. One Decipol represents pollution by one person ventilated by 10l/s of fresh air. Fanger (1988) also introduced an equation to measure the number of the dissatisfied occupants as a function of the perceived air pollution using Decipol units (Kosonen and Tan, 2004, Fanger, 1988). Above review of indoor air quality summarises four strands of research focus. These are air quality (constituents and pollutants), ventilation rate, the effect on human health, and occupant's perception and productivity. All of these aspects are interrelated and affect indoor air quality. Indoor air quality parameters and metrics are discussed in section 7.1. Thermal comfort is described as a "state of mind which expresses satisfaction with the thermal environment" (ASHRAE, 2004) . It is a subjective state which varies from person to person. The judgement of comfort is a cognitive process involving inputs influenced by physical, physiological and psychological factors (Lin and Deng, 2008) . Comfort is dependent on highly independent and dynamic factors such as clothing, altering activity, changing posture, window location and mood (ASHRAE, 2005). Achieving overall thermal comfort in a building is a complex task, as thermal comfort is an outcome of different physical parameters, creating a thermal state and understanding a collection of subjective human responses to that thermal state. Thermal comfort varies individually and geographically due to a broad range of factors such as age, sex, metabolism rate, time of the year, among many others (Cena and de Dear, 2001 , Quang et al., 2014 . Thermal comfort in an office is measured by analysing the number of discomfort complaints. Complaints analysis is a reactive method. By pursuing this research on IEQ, we can proactively design buildings that are more comfortable for occupants and lead to higher productivity. Literature uses thermal sensation, thermal acceptability and thermal preference to describe the human response to thermal comfort (Langevin et al., 2013) . Thermal comfort is a subjective condition for humans, but thermal sensation is an objective condition (Hensen, 1991) . Thermal sensation is defined as the direction and magnitude of one's sensory perception of the surrounding thermal environment (ASHRAE, 2010). The American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) define it on a seven point scale from -3 (cold) to +3 (warm). Thermal comfort is influenced by six factors. These factors are air temperature, air velocity, relative humidity, mean radiant temperature, clothing insulation and metabolic rate (Macpherson, 1962 , Macpherson, 1973 , Goldman, 1999 , Berglund, 1977 , Lin and Deng, 2008 . Thermal acceptability is the degree of an occupant's approval of the environment. Thermal preference is the ideal thermal state in terms of the environment (Langevin et al., 2013) . Occupant productivity increases when an environment moves from acceptable thermal state to preferred thermal state. However, preferred thermal state of an environment varies from person to person. Studies suggest that subjects with different age, gender and BMI (Body Mass Index) have different preferred thermal state (Tuomaala et al., 2013 , Indraganti et al., 2015 . Literature also indicate that females express more dissatisfaction than males in a thermal environment and are more sensitive to deviations from an optimal thermal environment and are less satisfied than males especially in cooler conditions (Karjalainen, 2012 , Karjalainen, 2007 . Achieving thermal comfort in a building requires energy consumption by the heating and ventilation systems in a building (Kwok and Rajkovich, 2010) . It has attracted many researchers focusing on sustainability in the built environment. Research on thermal comfort has been active for decades. The first instrumental work was by Fanger (Fanger, 1970 , Fanger, 1984 . Fanger (1970) developed an analytical model to predict thermal comfort. It combines four physical parameters (air temperature, mean radiant temperature, air velocity and relative humidity) with human variables (clothing insulation and activity level) to define the PMV (Predicted Mean Vote) index. The Predicted Percentage of Dissatisfied Occupants (PPD) is calculated from the PMV index (Fanger, 1984) . It predicts the percentage of people who could be dissatisfied with a thermal environment. It predicts if a large group of individuals are likely to feel 'too warm' or 'too cold', defined by voting +3, +2, -3, -2 on the scale (Olesen and Parsons, 2002) . This method uses climate chamber data and is based on the heat balance theory along with thermoregulation physiology (Charles, 2003 , Djongyang et al., 2010 ). An alternate approach known as the"adaptive approach" to thermal comfort was proposed by De Dear (De Dear et al., 1997, Brager and . It is based on field studies and analyses human acceptability of a thermal environment. It states that the thermal acceptability of occupants influences their thermal comfort. Thermal acceptability is dependent on adaptive behaviour, physiological adaption and psychological adaption. There are different thermal comfort standards and evaluation methods based on thermal comfort research (Olesen and Parsons, 2002 , ASHRAE Standard, 1992 , De Dear and Brager, 2002 , ASHRAE, 2005, ISO, 2005. These standards are used by industry professionals across the globe. These standards are developed for models and studies based primarily in North America and Northern Europe (Ogbonna and Harris, 2008). They are appropriate for static and uniform thermal conditions and do not consider human factors like age, gender and regional behavioural actions and expectations (Zhai et al., 2015 , Han et al., 2007 . These standards may not be entirely accurate for different regional climates and all type of indoor activities. The standards and thermal comfort prediction methods based on 'rational approach' that do not consider age, gender and race and suggest a narrow, welldefined thermal comfort range have been challenged and proven wrong by many research studies in different parts of the world (Ogbonna and Harris, 2008 , Nicol, 2004 , Humphreys, 1978 . The comfortable temperature of people is closely related to the mean temperature they experience. Field studies in UK, India, Iraq and Singapore indiciate that mean comfort temperature changes with the mean temperature experienced, especially in naturally ventilated buildings (Nicol et al., 2012 , Humphreys et al., 2007 , Nicol and Humphreys, 2002 , Humphreys, 1978 , Humphreys, 1975 , Nicol and Humphreys, 1973 . This variance in comfortable temperature highlights the significance of understanding the contextual climatic conditions and occupants' profile of the building. Research also indicates that clothing (acceptable formal dress) also influence the occupant (employee) comfort (Morgan and de Dear, 2003) . The formal dress code varies from culture to culture across the globe. This presents an opportunity to map a copmparision amongst various countries and their attire in line with acceptable comfortable temperature. Thermal comfort plays a significant role in occupant productivity. Dissatisfaction with thermal comfort leads to productivity loss (Roelofsen, 2015 , Akimoto et al., 2010a , Lan et al., 2011 , Fisk, 2000b . Studies indicate that temperature change within the 18°C -30°C range can influence the performance of office occupants in tasks like typewriting, learning performance and reading. The temperature range 21°C -25°C is a stable temperature range for office productivity. There is a decrease in occupant performance by 2% per 1°C increase in temperature in the range of 25°C -30°C (Seppanen et al., 2003, Seppänen and Fisk, 2006) . Ambient temperature plays a significant role in defining the thermal comfort of occupants, but local thermal conditioning or task conditioning also influence occupants' comfort levels. Task conditioning systems aim to maintain thermal conditions for a localised zone, and they are controlled individually or by a group of occupants (Zheng et al., 2009a, Bauman and Arens, 1996) . They are utilised based on evidence that indicates that personal control over thermal conditioning systems helps to improve productivity (Bauman et al., 1998 , Akimoto et al., 2010b , Seppänen and Fisk, 2006 . Local thermal conditioning and personal control is highly applicable in office with employee groups seated in different zones. It is also applicable in office that involve employees to work in various task demanding different levels of physical activites. While, thermal control amongst employee is associated with higher comfort and satisfaction, it also raises certain problem related to the control behaviour of the occupants. Acceptance of a thermal enviornment is subjective and directly related to the expectation of the occupant. There are individual differences in user's comfort expectation and expectation from the enviornment (Moezzi, 2009 ). This could result in energy wastage due to inefficent use of thermal control system (Shahzad et al., 2016) . Energy wastage due to inefficent control of thermal control system in offices is related to lack of knowledge of system operation and general thermal comfort disagreement amongst occupants. Providing energy awareness can cause behavioural change towards more energy concious operations and may result in energy saving along with occupant comfort and productivity (Tetlow et al., 2012 , Nisiforou et al., 2012 . Innovative building energy and comfort control strategies must be developed and implemented to adjust occupant behaviour and devices' control. A real world interface with strong computational support might help in this context Research on the relationship between office tasks, temperature and productivity indicates that different functions have different optimum temperatures of productivity , Tanabe et al., 2007 . Optimal thermal comfort and optimum performance may not coincide with the particular type of tasks. For example, creative mental work may have the optimal thermal temperature and optimum performance coinciding. However, another kind of mental work may require a slightly cold temperature for optimum/increased productivity (Fisk, 2000b) . These findings highlight the importance of focusing on offices tasks when determining the thermal design for an office. Application of local control system in different task zones of an office would help to maintain an overall occupant comfort and productivity. Thermal comfort measurement parameters and metrics are discussed in section 7.2. Daylight influences our day-to-day tasks throughout the year. Daylight controls our biological clock/internal clock which has been set for millions of years according to the rising of the sun and the darkness of night. This biological clock has been passed on by our ancestors. We need light as it is a regulator of our physiology and performance (Aries, 2005) . Office employees spend most of their time indoors thus office tasks are dependent on indoor lighting or any available daylighting. Daylight is considered to be the best source of light with excellent colour rendering that offers the best light for human visual comfort. It provides a sense of cheeriness and brightness and has a positive impact on people (Li and Lam, 2001, Li, 2010). Buildings worldwide contribute to roughly 40% of the world's annual energy consumption (Omer, 2008) . Lighting has the highest share (33%) in the calculation of average electricity consumption in the UK office buildings . In the USA, commercial buildings account for one-third of the nation's primary energy consumption. Artificial lighting utilises 25-40% of the total commercial buildings' energy consumption (Krarti et al., 2005) . In Spain, lighting contributes to 33% of total energy consumption in offices (Pérez-Lombard et al., 2008) . This highlights the occupants' dependence on artificial lighting in an office. It also indicates that artificial lighting is a major contributor to carbon dioxide emissions and global warming as it utilises one-third of the electricity bill (Busch et al., 1993) . Efficient daylighting measures and intelligent lighting systems can help reduce the artificial energy load and thus the carbon footprint of a building. Organisations that pay attention to the importance of daylighting achieve higher occupant productivity in their workplaces (Yang and Nam, 2010 , Fay et al., 2002 , Ander, 2003 . Companies like Lockheed Martin and VeriFone have reported a 15% decrease in absenteeism and a 47% increase in attendance, respectively, in buildings designed to provide maximum daylight for their occupants (Romm and Browning, 1994) . Building occupants prefer natural light/sunlight over artificial light (Boyce et al., 2003 , Elzeyadi, 2011 , Galasiu and Veitch, 2006 , Leslie, 2003 , Guzowski, 2000 , Reinhart, 2002 . Occupant preference is due to physical, physiological and psychological reasons. Physically, there is not much difference as artificial lamps can cover the spectrum and colour rendering of daylight at a certain cost. However, the literature highlights that physiological and psychological reasons affect the visual and circadian system in humans . Human visual system parameters such as visual size, luminance contrast, colour difference, retinal image quality and retinal illumination jointly affect the work performance of any task (Boyce, 1997). Natural light's levels are above the order of magnitude of normal electric lighting levels. They influence melatonin, a hormone responsible for regulating the body's internal clock and these light levels (Rea et al., 2002, Van Bommel and Van den Beld, 2004) . It relates to the finding that natural light is believed to increase alertness for monotonous work during office hours (Boyce, 1997 , Nagy et al., 1995 . These studies demonstrate the importance of natural light for the human body to work efficiently in daily activities. A study of office workers focusing on the importance of windows and their benefits reports that almost 99% believed that offices should have windows and that 86% considered daylighting to be the preferred source of light for office tasks (Cuttle, 1983) . Windows also provide views with different visual information like weather, nature and surrounding activities. These views could be aesthetically pleasing and could have a positive impact on the occupants' productivity Orians, 1984, Heerwagen and Heerwagen, 1986) . The literature confirms occupants' liking of windows. However, the acceptability of window size varies depending on factors such as the amount of indoor and outdoor light levels, sun position and sky luminance (Ne'Eman, 1970) . Studies focusing on window sizes and glazing highlight a broad spectrum of window size preferences among occupants and it is not necessarily the case that all the occupants universally prefer a large window size. A study reports that only 43% of respondents prefer large windows for an office environment, and different types of spaces have different illumination requirements (Butler and Biner, 1989) . A case study focusing on high-rise office buildings reports that only 56% of the respondents believed in the importance of window access. The size of the window was unimportant to 86% of the workers as long as the light levels were satisfactory (Wotton, 1982) . The orientation of the window produces distinctly different control responses due to solar radiation and wind direction (Zhang and Barrett, 2012b) . A glazed building's southern side uses more blinds than on the northern side, indicating the occupants' disliking of an excess of direct sunlight (Rubin et al., 1978) . Solar altitude and solar radiation plays a huge part in blind usage in windows, indicating annoyance created by excessive sunlight (Zhang and Barrett, 2012a). Window control is highly correlated with outside air temperature, the season of year, time of a day and the occupancy pattern of the office (Zhang and Barrett, 2012b). Geographical location, local climate and immediate surrounding plays and important role in defining the size, position of the window or type of glass façade. If efficiently designed, glass façade is applicable in Hong kong's sub-tropical climate and closely packed buildings to harvest adequate sunlight (Li and Tsang, 2008) . However, the inadequate design and misuse use of glass façade in high towers of Dubai without considering orientation and solar attitude leads to occupant annoyance and use of blinds (Aboulnaga, 2006) . The use of blinds leads to use of artificial lighting and wasting available sunlight and increase in CO2 emissions. External design features in façade can be used to avoid glare problem even in the hot and high solar radiation areas (Lim et al., 2012) . Traditional dwellings in India use 'jaali' (latticework on stone or wood) in windows in tropical monsoon, arid (steppe) and arid (desert) climate areas as passive design strategy to maintain ventilation and temperature along with adequate daylight without any glare (Srivastav and Jones, 2009) . Similarly, designers use double façade to reduce heat gains and glare problems from direct sunlight in hot climatic. The use of traditional mashrabiya' screen as second skin in Masdar city project in Abu Dhabi and screen pattern on Doha tower in Abu Dhabi are great examples of reducing heat gains and glare problem from direct sunlight in hot climate (Boake et al., 2014) . The daylight design of a building should be designed according to local climate, building's orientation, solar altitude, and immediate surroundings, nature of the space, and layout of the building alongside and daylighting availability. The above research indicates how people in different geographical regions have developed native architectural practices for lighting. A daylighting design can be any method by which natural light is brought into a room to provide adequate lighting conditions. It involves analyses of daylight availability which is dependent on the light available from the sun and the sky at a certain locations, times and weather conditions (Guzowski, 2000) . Daylight analysis lays the path to the daylighting strategies of side-lighting (windows) and top-lighting (skylights, roof monitors) (Manning, 2006) . Illuminance from natural sources is determined by the Daylight Factor (DF). The Daylight Factor is the percentage of outdoor light under overcast skies available indoors. It takes three components into account; the sky component (SC), the light from external surfaces/externally reflected component (ERC) and the light reflected from surfaces within the room/internally reflected component (IRC) (Fontoynont, 2014 , Reinhart et al., 2006 , Hopkinson, 1963. The recommended level of Daylight Factor (DF) for different offices depends on the type of office task. Simple tasks like reading, filling only require 1.5 -2.5% daylight factor while draughting and fine machine work requires up to 8% daylight factor (Stein et al., 1992) . Excess of direct sunlight and artificial light can result in a glare that results in visual discomfort. A low level of lighting can lead to ocular discomfort and improper lighting design leads to worker dissatisfaction. Glare-free and thermally comfortable spaces have a high effect on occupant productivity (Selkowitz, 1999 , Abdou, 1997 , Hemphälä and Eklund, 2012 . The Illuminating Engineering Society of North America (ILESNA) describes the density of luminous flux on a surface, which is represents flux per unit area of distribution. (Rea, 2000) . A high level of illuminance from daylight may cause glare inside a building, called Daylight Glare (Rea, 2000) . Daylight glare has the measurement metric known as Daylight Glare Index (DGI). Office areas are recommended to have 22 DGI for good visual comfort (Cantin and Dubois, 2011, Hopkinson et al., 1966). Glare can be caused by combination of daylight and artificial lighting. Lighting control mechanisms can be used to control it.There are two types of smart daylighting control to reduce a building's energy consumption (Doulos et al., 2005 , Chang and Mahdavi, 2002 , Sullivan et al., 1994 . These are:  Continuous dimming -This strategy uses a sensor that reads the exterior illuminance and controls the artificial lights inside a building to maintain a set indoor illuminance level. This technique is highly dependent on the location and hardware quality of the sensor. This is strategy also uses automated shading devices in responsive façade design to efficiently manage the overall illuminance levels of the indoor space. Al Bahar towers in Abu Dhabi are great example of managing high level of direct sun using responsive façade structures to manage solar gains and excessive daylight (Boake et al., 2014) . This strategy would be also successful in climatic areas with high level cloud movements that change outdoor lighting levels.  Switching control -Switching loads on and off manually or with relays controlled by occupancy sensors. It is a more rudimentary strategy and becomes less efficient when daylight levels fluctuate more rapidly. Lighting and Daylighting measurement parameters and metrics are further discussed in section 7.3. The hearing ability is one of the five human senses. Varieties of sound regularly reach our ears and get transmitted to our brain. The World Health Organisation (WHO) describes any unwanted sound as noise. The effects of noise in terms of an auditory response can be described in psychological attributes such as annoyance, noisiness and loudness (Berglund et al., 1996 , Stansfeld and Matheson, 2003 , Ayr et al., 2003 . Magnitude can characterise sound (or noise). It is measured on a log scale of decibels (dB), sound power and sound pressure levels (SWL and SPL). Noise and acoustics have high relevance in building design. A building's acoustic performance becomes more important in office design as most office tasks require a degree of noise control to enable the occupants to work efficiently. Bad acoustic and noise performance can lead to dissatisfaction with the office environment and can affect workers' performance (Frontczak et al., 2012 , Balazova et al., 2008 , Toftum et al., 2012 . Noise in an office can have two locations for sources; external or internal. External sounds include traffic, the public, air traffic, machinery. Internal noises include co-worker conversations, one-minute requests, machine sounds such as telephones and other office equipments (Banbury and Berry, 2005, Ayr et al., 2003). The neutral sound pressure of a typical air-conditioned office is between 45 dB and 70 dB (Mui and Wong, 2006) . Occupant discomfort is an outcome of different Indoor Environment Qualities (IEQ). Acoustic sensation and thermal sensation have an equivalent effect on an employee. A temperature change of 1°C has the same effect on productivity as a change in noise of 2.6 dB (Pellerin and Candas, 2004) . Continuous and prolonged noise at higher levels can induce and increase stress levels over time. Studies suggest that occupants subjected to continuous transport/traffic noise showed higher levels of blood pressure and stress hormones (Evans et al., 1998) . Internal machinery such as fax machines, telephones and air conditioning systems can also produce annoyance that may result in similar stress levels (Ayr et al., 2003) . Office-related tasks are highly affected by speech and office noise. In a study, researchers experienced a drop in performance by 66 percent in 'memory for prose' tasks when participants were exposed to different types of background noises. About 99 percent of the people surveyed in an experiment reported on their concentration being impaired by office sounds such as unanswered phones and background speech (Banbury and Berry, 2005) . Both internal noise and external noise affect occupant's performance and leads to stress and anxiety and possibly creating long term health issues for occupants. External building elements and design can protect the office interior from external noise. Internal arrangement and office layout influence the internal noise in the office environment. Open plan office noise can have a negative impact on the fatigue, motivation and performance of employees (Jahncke and Halin, 2012) . Tasks associated with word processing and numbers' calculation are affected by internal office noise. Open plan office employees are more prone to privacy issues and disturbances due to the various office sounds around them (Toftum et al., 2012 , Balazova et al., 2008 Acoustic comfort can also be achieved using sound absorbing materials in the room. Acoustic quality of a room is described by the reverberation time and the sound absorption of a room. It is related to the absorption characteristics of the room surfaces and the volume of space (Payne, 2013) . Privacy may be a concern in a cubicle/cellular based office arrangement. Hence, good insulation and moderate background noise may be required to mask the speech in cubicles. Architects can use office layout, materials selection of internal partitions and external envelope to reduce acoustic discomfort of an office environment. Noise and Acoustics measurement parameters and metrics are discussed in section 7.4. Office layout design is one of the leading indoor qualities that influence workplace performance and behaviour. Design, proximity and privacy influence our work pattern and performance (Haynes, 2008b , Lee, 2010 . The physical environment of the workplace influences the recruitment, retention and productivity of the organisation, thus affecting an organisation's business capability to achieve success . The office layout of an organisation should be well designed to ensure efficient work process to enable organisational success. The literature reviewed iterates the debate and discussion on office layouts and their effect on occupant productivity. It highlighted two key themes: the issue of an open office versus cellular offices and their relationship to the office work processes (Stallworth Jr and Kleiner, 1996, Haynes, 2008b). Many pieces of research highlight that a mismatch of the office environment and an organisation's work process leads to productivity loss (Mawson, 2002 , Laing et al., 1998 . To understand work patterns more efficiently, Liang proposed four metaphors for office environments that reflect the work processes of an organisation. These are Hive, Cell, Den and Club. Liang emphasised the importance of understanding the office work structure when designing an office layout (Laing et al., 1998) .  Hive: This is a combination of cellular or combi offices which includes both cellular and open plan. This is ideal for organisations that have individual routine work which requires low interaction and high individual work. These are the organisations where majority of the work can be done at a simple workstation during regular working hours.  Cell: The use of cellular offices is generally suitable for organisations which have a high level of concentrated individual work with barely any interaction requirement. Individuals occupy the office at irregular time patterns based on requirements.  Den: This type of layout is suitable for organisations which require high level of group working with a low level of individual work.  Club: This type of layout includes combi offices along with hot desking facilities. This is suitable for organisation which is involved in knowledge work. It is interactive and autonomous in operation. Occupancy pattern is intermittent and extends over usual hours. These office environment models represent a variety of work processes in an office and how an office environment can accommodate different processes by providing different layouts. The suitability of an office layout is also affected by the complexity of employees' tasks. Task complexities have shown to affect employee performance and reaction to workspace (Maher and von Hippel, 2005, Stone, 2001). An Office layout that compliments work process of an organisation can help the workflow to efficiently stream through the office reducing time and improving productivity. Office layout is a physical environment that influences the behaviour of the office occupants. Haynes (2007a) provides evidence supporting the fact that distraction is the component that has the most negative impact on perceived productivity and interaction has the most positive effect on perceived productivity. Office workers in most organisations wish to have distraction-free solo work and opportunities to have informal interaction with colleagues (Haynes, 2007b , Roper and Juneja, 2008 , McCoy, 2002 , Heerwagen et al., 2004 , Chachere et al., 2003 . The physical setting and layout of the workspace is part of organisational culture. Literature indicates that physical setting and artefacts play a crucial role in defining and changing organisation's culture (Hatch, 1993 , Kallio et al., 2015 , Lindahl, 2004 , Lamproulis, 2007 , Barclay and York, 2001 ). The design of physical environment plays an implicit yet significant role in developing a culture that promotes organisational creativity and knowledge sharing. It affects equality, openness and collectivity of an organisation's culture (Kallio et al., 2015) . However, national culture also plays significant role organisation's culture and office layout. A study highlights that Arab national culture's emphasis on trust, 'wasta' (use of personal connections), status, power reflects in the office design. Lack of openness, equality of space and informal gathering area are the direct implication of Arab culture's high collectivism (Al-Esia and Skok, 2015) . This reflects that office design and layout is highly affected by external factors, and it will be influenced by organisational and national cultures.The literature suggests that occupants feel more distracted in an open plan office as compared to a cellular office (Brennan et al., 2002) . Distraction frequency was found to be the highest in an open-plan office (65%) followed by double-room occupancy (53%) and was least from single-room occupants (29%) (Brill and Weidemann, 2001 ). The open plan office also reports satisfaction inversely related to proximity to a window. Employees experience a lack of visual and acoustic privacy and feel satisfied with high partition and a window. It is also noted that female exhibit higher dissatisfaction than male counterparts (Yildirim et al., 2007) . Office Layout measurement analysis and metrics are discussed in section 7.5.
Based on the literature survey conducted in REF , it was observed that building occupants prefer natural light/sunlight over artificial light and daylighting achieved higher occupant productivity, hence it is important to consider daylighting in any lighting control system.
Occupant productivity and office indoor environment quality: A review of the literature
Building and Environment
Engineering