It has been established that a wide range of indoor air contaminants negatively affect both human health and IAQ (O.S.H.A., 2020). Particulate matter (PM2.5, PM10), Nitrogen oxides (NOx), volatile and semi-volatile organic compounds (VOCs), Sulphur Dioxide (SO2), Ozone (O3), Carbon monoxide (CO), radon, hazardous metals, and microbes are the principal indoor air pollutants. Table 2 enumerates some prevalent contaminants.

Even at low air pollutant concentrations, prolonged exposure to indoor anthropogenic activities can degrade IAQ and pose serious health hazards to people. The scientific community and pertinent organizations have tried to create and implement IAQ standards and recommendations to address these IAQ issues (Tran and Park, 2020). The international community finally succeeded in establishing IAQ standards and guidelines based on an integrated building strategy after much effort (Aristotelis Avgelis, 2016). The aim is to eliminate, or at least reduce, potential populations because the WHO and USEPA(United States Environmental Protection Agency) state that the purpose of IAQ guidelines is to provide a vital database as a reference for the prevention of detrimental consequences of IAP and preservation of public health (W.H.O., 2000). The WHO's Air Quality Guidelines (AQG) for a few prevalent contaminants are presented in Table 3 (W.H.O., 2021). The WHO and USEPA standards often specify the maximum concentration for duration (e.g., one hour, twenty-four hours, or a year).

IAQ is affected by the concentration of total volatile organic compounds (TVOCs), which are made up of many individual chemical species that come from indoor sources like furniture and building materials. Different international guidelines rely on qualitative ratings of air quality to determine permissible TVOC levels. For instance, the North American LEED (Leadership in Energy and Environmental Design) and RESET (Regenerative, Ecological, Social & Economical Targets) guidelines specify that TVOC concentrations should not exceed 500 μg/m3 to protect health. On the other hand, more stringent standards enforced by the German Federal Environmental Agency and Chartered Institution of Building Services Engineers (CIBSE) TM40 - 2020 Guide – Indoor air quality comparisons recommend that total VOCs be kept below 300 μg/m3 to promote better IAQ and reduce exposure. By continuously measuring and reducing indoor TVOC concentrations in comparison to these reference values, indoor environments can effectively reduce the potential health risks associated with indoor air pollutants. Table 4 shows the exposure limits of TVOC set by CIBSE, LEED, RESET, and IAQ Levels by the German Federal Environmental Agency.

The index used by the USEPA for reporting air quality is called the Air Quality Index (AQI) (AirNow, 2023). The amount of air pollution and the corresponding health concerns increase with a higher AQI value. For instance, good air quality is indicated by an AQI value of 50 or less, whereas hazardous air quality is indicated by an AQI value of 300 or higher (AirNow, 2023)). An AQI value of 100 for any given pollutant typically denotes a level of ambient air concentration that meets the national ambient air quality standard for public health protection for a given period. Air quality becomes unhealthy when AQI values are above 100, initially for sensitive individuals and later for everyone as the numbers rise (AirNow, 2023). According to most research, PM 2.5 levels of 12 μg/m 3 or below are thought to Extended exposure to concentrations higher than 50 μg/m3 can cause major health problems and early death (Indoor Air Hygiene Institute, 2023). The air quality standards set by EPA for PM10 and PM2.5 are shown in Table 5 (AirNow, 2023).

The WHO has imposed a tougher eight-hour limit than both OSHA and NIOSH, at 100 μg/m³ (0.1 mg/m³) for O3. The EPA's AQI breakpoints provide an additional lens through which to view ozone. Although we are using these breakpoints for ambient ozone, we can also use them to calculate safe indoor ozone levels (Kaiterra, 2021). Ozone exposure is divided by the EPA into two-time intervals: one hour and eight hours. The following Table 6 lists different ozone concentrations along with how safe they are to be exposed to for eight hours.

As per the current WHO air quality recommendation, an indoor nitrogen dioxide guideline of 200 μg/m3 for one hour is advised (W.H.O., 2020). Asthmatics show slight reductions in lung function at roughly twice this level. At this stage, sensitive individuals may already exhibit slight alterations in their airway's reactivity to a range of stimuli. There is no evidence to support an indoor guideline that differs from the ambient guideline from studies on the indoor environment (W.H.O., 2020). As per the current WHO air quality recommendation, a yearly average indoor nitrogen dioxide guideline of 40 μg/m3 is advised (W.H.O., 2020). Table 7 shows the US EPA range for NO2 and their AQI categories.

Higher or longer exposure levels have the potential to cause death via airway constriction, but exposures of 50 to 100 ppm may be tolerated for more than 30 to 60 minutes. Because sulfur dioxide is heavier than air, it can cause asphyxiation if it is present in enclosed, poorly ventilated, or low-lying places. The WHO has not identified SO2 as a pollutant for which specific IAQ guidelines are required, The WHO guidelines are for general air quality (W.H.O., 2021). There are no established CO standards for indoor air (U.S.E.P.A., 2023). Any home with a fossil fuel appliance will contain carbon monoxide (CO), also known as "The Silent Killer," a colorless, odorless gas. Even though most poisonings happen in the winter, CO can exist year-round (McAfee, 2021). The following details in Table 8 describe some symptoms that could manifest after one hour of CO exposure:

Hazardous VOCs, like butanol, formaldehyde, and acetone, are present in a lot of household items and can pollute indoor air. It is crucial to understand VOC concentration and units of measurement to protect public health. For instance, common harmful substances include:

Within the confines of human indoor environments, furniture assumes a substantial role. Furthermore, one must comprehend both the emissions of building materials and furniture to assess the true amounts of VOCs indoors. Wood-based panel, adhesive, and surface coating materials are the primary components of furniture, and these materials can release a variety of VOCs, such as aldehydes, terpenes, aromatic hydrocarbons, esters, ketones, hydrocarbons, and so on (He et al., 2012). The focus of furniture research is on VOCs and the amounts of these emissions (Kang et al., 2017). Ho et al. (2011) and (Song et al., 2015) identified 39 target VOCs from five widely accessible furniture brands. The findings demonstrated that the VOC contents differed significantly amongst the products. More than 400 different types of VOCs were found in the new dorms that (Pei et al., 2016)) evaluated for long-term indoor gas pollution. The dormitories had recently been furnished. According to (Chang et al., 2017), panel furniture was the primary source of TVOC and formaldehyde indoors. Moreover, it has frequently been determined that the benzene series comprises most furniture components (Song et al., 2015). Ultimately, the amount of VOCs released by furniture is mostly determined by the constituent materials used in its manufacture, its kind and age, the production method, and how it is stored, transported, and utilized indoors (Yan et al., 2018).

In a study conducted by (Yan et al., 2018) two different types of furniture (a footstool and a bedside table) had their VOC emissions measured in an environmental chamber at three different loading rates. The most common substances found in the footstool and bedside table were, respectively, n-undecane and styrene. VOC concentrations rose swiftly, peaked in about 1-2 hours, and then declined as emission rates reduced. In a study by (Ho et al., 2011), five furniture samples - a desk chair, bedside table, dining table, sofa, and cabinet were used to quantify the VOC emission rates from common furniture used in homes and businesses during a period of up to 14 days following the two weeks following the furniture's creation. The findings indicated that the predominant components of emissions were toluene and α-pinene, with most VOCs showing comparable declining tendencies with time. If measured in terms of VOCs, the relative ordering of emission rates for each of the five types of furniture remained relatively constant over time: dining table > sofa > desk chair > bedside table > cabinet.

VOCs can be harmful to both human and animal health. Unfortunately, typical household materials such as some paints, air fresheners, and cleaning supplies release them. When paint is applied, VOCs are gradually released over several weeks or months. At least 48 hours following painting, a space may be deemed "high risk" for volatile organic compounds. Precautionary steps ought to be followed for a full 72 hours including staying away from the room and not sleeping in it (Ghobakhloo et al., 2023). The duration of VOC emission following painting is contingent upon several elements, such as the type of paint used, the space in which it is applied, ventilation, the existence of an air purifier, temperature, humidity, and numerous others. Paint-related VOCs do eventually evaporate when the paint dries on the wall, but this process can take some time, with the majority dissipating during the first six months of application (Ghobakhloo et al., 2023).

Many common insulation products are used in the construction of off-gas volatile organic compounds (VOCs) into indoor air, degrading air quality and posing health hazards. VOCs are gases containing elements like carbon, oxygen, and nitrogen emitted from solids and liquids. Insulation materials made from plastics, polymers, and some synthetic fibers release VOCs such as formaldehyde, benzene, toluene, xylenes, and styrene during and after installation (Adamová et al., 2020). Exposure to these chemicals can lead to eye, nose, and throat irritation, headaches, breathing difficulties, nausea, kidney and liver damage, and cancer risks. As buildings aim to cut energy use through increased insulation, understanding these unintended emissions consequences is vital.

Controlled lab analysis has allowed the quantification of VOC emission rates from various insulation materials over time. In a study conducted by (Wi et al., 2021) on a 20 m2 test bed, five different types of insulation materials were developed, and the pollutants' concentrations were tracked over time. Five different types of building insulation were chosen to serve as the test groups: two types of PF insulation, one type of XPS, one type of EPS, and one type of PIR. Through the simulation of unfavorable climatic circumstances, this study attempted to analyze rather clear changes in IAQ. The MOLIT's health-friendly housing guidelines state that indoor building finishing materials, like flooring and wallpaper, have TVOC emissions of no more than 0.10 mg/m2 and HCHO emissions of no more than 0.015 mg/m2 (Wi et al., 2021). The MOE has issued the IAQ management legislation enforcement regulations, which include emission restrictions for building materials. Specifically, the TVOC emission limit for flooring materials and wallpapers is 4.0 mg/m2, while the HCHO emission limit is 0.02 mg/m2. The Korea Air Cleaning Association (KACA) has devised a collective certification system for healthy building materials. The best rating for TVOCs and HCHO is less than 0.10 mg/m2 and 0.008 mg/m2, respectively (Wi et al., 2021). Table 9 displays the TVOC emission data from (Wi et al., 2021) utilizing the 20 L small chamber. Based on the insulation, the largest TVOC emissions were for PF- 2 at 0.1727 mg/m2 -h, and for EPS at 0.1216 mg/m2 -h. The emissions from PF-2 and EPS were less than those required by the emission regulations of building materials for flooring and wallpaper in IAQ management (MOE). They did, however, surpass the healthy building certification requirements (KACA) and the health-friendly housing construction standards (MOLIT) (Wi et al., 2021). Table 10 shows the properties of mineral and bio-based thermal insulations. Among them, Hemp insulation is the one with no known pollution and has no known detrimental effects on health. Hemp insulation materials (HIMs) have gained attention for their environmental benefits (Martínez et al., 2022). A study explored the feasibility of large-scale hemp cultivation in Canada and the suitability of HIMs for residential buildings (Liu et al., 2023). It is argued that full substitution using 5% hemp fiber insulation (HF) and 95% hempcrete (HC) can mitigate 101% of greenhouse gas emissions caused by existing mainstream insulation materials (MIMs), contributing to a 7.38% reduction in emissions and aiming for net-zero emissions by 2050 (Liu et al., 2023). Additionally, hemp-based materials offer lower embodied carbon compared to fossil fuel-based alternatives (Martínez et al., 2022). Case studies have demonstrated the effectiveness of hemp insulation in realworld applications such as historic building retrofits (Johansson et al., 2018),(International Hemp Building Association, n.d.)).

Air leakage, defined as the normal unintended movement of air into and out of buildings, is an important contributor to building energy loads that is often overlooked (Chan & Joh, 2013). The leakage rate is measured in air changes per hour (AC/H), which quantifies the rate of replacement of internal air with external air. Temperature differences between indoor and outdoor environments, as well as wind pressures on the building envelope, can increase air leakage rates considerably. A simulation analysis of a wide range of residential and commercial building types by (Chan & Joh, 2013) demonstrated the major impact that air leakage can have on energy consumption. By reducing leakage through cost-effective measures such as applying sealants, installing weatherstripping around windows/doors, adding storm windows, or replacing worn-out components, the study found heating and cooling energy use could be lowered by 5-40% (Chan & Joh, 2013). The large range represents differences in building construction and leakiness levels.

It is commonly observed that retrofitted buildings do not achieve the estimated energy savings that were computed to guide the design phase or to get energy performance certification (Sunikka-Blank & Galvin, 2012) (Brom et al., 2017). The "energy performance gap" refers to the discrepancy between the projected (or simulated) and measured (or actual) post-retrofit energy performance of buildings. Most studies assess the energy performance in terms of the building's annual energy demand for heating and/or cooling (Wilde, 2014). There are signs that dwellings refurbished using the aforementioned weatherisation procedures tend to deteriorate IEQ and contribute to ill health (Bone et al., 2010) (Richardson & Eick, 2006), as well as may lead to energy performance gap resulting in missed governmental targets, increased energy bills, and longer investment pay-back timelines (Anastasios & Itard, 2018) (Majcen, 2016). According to the European Commission's policy report from 2016, a lack of information and statistics on IEQ in energy-efficient homes may jeopardize inhabitants' health and comfort (Kephalopoulos et al., 2016).

Controlling airtightness, together with thermal insulation of the building envelope and its windows, has been identified as a critical method for achieving energy savings in buildings. This is because space heating accounts for more than half of all carbon emissions in the residential sector. As a result, buildings that allow air seeping and/or heat loss are likely to spend more on heating (Pan, 2010). Weatherization and restoration programs that are heavily focused on lowering permeability and boosting thermal insulation to save energy can introduce some dangerous factors to the inhabitants' health (Ortiz et al., 2020). Internal and surface condensation, moisture surplus or dampness, pollutant buildup owing to limited ventilation, radon concerns, and overheating may come from adding internal thermal insulation and increasing building airtightness (Ortiz et al., 2020). These IEQ problems also Narayanan/ Environmental Science and Sustainable pg. become more severe when the mechanical ventilation systems are not correctly planned, installed, maintained, or operated (Ortiz et al., 2020).

When sealing the home for energy savings, the indoor chemistry of the residence can have a severe impact on the health of the occupants (J, 2011). Indoor contaminants can become more frequent in an airtight home, in addition to undesired emissions from the insulation components used (Marlow et al., 2014). In France, for example, a comparison of the IAQ of energy-efficient houses to conventional structures revealed greater concentrations of terpenes and hex aldehyde, probably due to wood or wood-based goods and human activities (Derbez et al., 2018). Thermal retrofit of dwellings and the absence of appropriate ventilation systems are associated with elevated levels of formaldehyde, toluene, and butane indoors (Yang et al., 2020).

The authors have received ethics approval from the ethics committee of the University of East London, for the interviews and surveys that were conducted by the authors.

The authors declare that there is no competing interest.