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Indoor Air Quality: Comparison
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Please note this is a comparison between Version 2 by Vicky Zhou and Version 1 by Sami G. Al-Ghamdi.

Along with the penetration of outside air pollutants, contaminants are produced in indoor environments due to different activities such as heating, cooling, cooking, and emissions from building products and the materials used. As people spend most of their lives in indoor environments, this has a significant influence on human health and productivity. Despite the two decades of indoor air quality (IAQ) research from different perspectives, there is still a lack of comprehensive evaluation of peer-reviewed IAQ studies that specifically covers the relationship between the internal characteristics of different types of building environments with IAQ to help understand the progress and limitations of IAQ research worldwide.

  • indoor air pollution
  • residential indoor pollutants
  • office indoor pollutants
  • school indoor pollutants
  • influencing factors indoor

1. Introduction

Research on the urban population has confirmed that people spend more than 90% of their daily lifespan in indoor environments. Apart from residential indoor environments, people spend a large proportion of their time in offices, educational institutes, and other different commercial and industrial buildings. Specific research in North America has shown that adults tend to spend 87% of their time in buildings, and the remainder of their time is spent in vehicles (6%) and outdoors (7%) [1]. As people spend a majority of their time in indoor environments, exposure to indoor air pollutants has a significant impact on both human health and effectiveness in the workplace. However, research on air quality has mostly focused on the outdoors, whereas indoor air quality (IAQ) and its impacts have received considerably less attention until the last decade [2]. Recently, both scientists and the public have focused on risks associated with IAQ because research has established that indoor air is more contaminated than outdoor air [3]. Due to continuous changes in living style and the materials used in indoor environments, there have been significant changes in terms of the nature and complex compositions of indoor air pollutants, which opens up avenues that need to be investigated in detail.

2. IAQ Standards & Assessment Methods

The quality of indoor air is crucial because people spend a significant portion of their time in different indoor spaces and also because of the presence of numerous pollution sources in indoor spaces, such as traditional and newly developed building materials, finishing products, furniture, cooking systems, and cleaning agents. Therefore, several international organizations worldwide, such as the WHO, have set guidelines and threshold values to maintain an optimal IAQ (Table 1). Apart from the WHO, the most recognized organizations involved in IAQ regulations include the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), US EPA, National Health and Medical Research Council in Australia, Health Canada, State Environment Protection Agency in China, Hong Kong Indoor Air Quality Objectives, Danish Society of Indoor Climate, Finnish Society of Indoor Air Quality and Climate, and Singapore Indoor Air Quality Guidelines [4].

Table 1. Standards for indoor air quality (IAQ) by international organizations/Government.

Parameters CAS WHO [5] Singapore [6] NIOSH [7] Canada [8] China [9] UK [10] Australia [11] US EPA [12]

Common IAQ measurement techniques.

Sampling Item Sampling Methods/Tools Sampling Duration/Cautions Ref.
Benzene (C6H6) 71-43-2 Q-Trak monitor (TSI Inc.): Nondispersive infrared analyzerNo safe level of exposure can be recommended Sampling duration: 7 days, 10 min (min) average- [13][6][8][14- - 90 ug/m3

[1 h avg.]		 - - -
][15] Carbon Di-oxide (CO2) 124-38-9 100 mg/m3 (15 min)

35 mg/m		3 (1 h)

10 mg/m		3 (8 h)

7 mg/m		3 (24 h) 1000 ppm

(8 h avg.)		 5000 ppm (8 h avg)

30,000 ppm (15 min)		 ≤6300 mg/m3

(≤3500 ppm)		 1000 ppm

(daily avg.)		 15,000 ppm (15 min avg.)

5000 ppm (5 min avg.)		 30,000 ppm

Integrated data loggers (Hobo HO-8) Sampling in every 5 min(15 min avg.) [800 ppm
16] Carbon mono-oxide (CO) 630-08-0 86 ppm (15 min avg.)

51 ppm (30 min avg.)

25 ppm (1-h avg.)

8.6 ppm (8-h avg.)		 10 mg/m3 (9 ppm)

(8 h avg.)		 35 ppm

(8 h avg.)		 ≤11 ppm

(8 h avg)

≤25 ppm

(1 h avg.)		 5.0 mg/m3

(daily avg.)		 11.6 mg/m3

(8 h avg.)		 9 ppm

(10,000 μg/m		3)

(8 h avg.)		 35 ppm

(1 h)

Indoor air quality meter (IAQ-CALC model 7545) NA [17]9 ppm

(8 h)
Formaldehyde 50-00-0
CO Electrochemical sensor (Draeger Pac III)

FIM CO- Tester Tx for exhaled air		mg/m3 (30 min)

0.2 mg/m		3 (long term) 0.1 ppm (120 μg/m3)

(8 h avg.)		 0.016 ppm

0.1 ppm (15 min)		 120 µg/m3 0.12 mg/m3

(1 h avg.)		 2 ppm (15 min avg.)

(2500 μg/m		3) 2500 μg/m3

(15 min avg.)		 920 μg/m3

(8 h)
Sampling duration: 7 days, 5 min average [18 Naphthalene 91-20-3 0.01 mg/m3 (annual avg.) - - - - - - -
]
NO2 Passive samplers (Palmes tubes) containing triethanolamine absorbent and analyzed by a spectrophotometer NA [19][20] Nitrogen dioxide 10102-44-0
PM10200 μg/m3 (1 h)

40 μg/m		3 Dust-Trak air monitor (Model 8520, TSI Inc.), Light scattering

(annual avg.)		 - 1 ppm (15 min) ≤100 µg/m3

≤480 µg/m		3 (1 h) 0.10 mg/m3

(daily avg.)		 200 μg/m3 (1 h)

40 μg/m		3 (1 year) - 0.053 ppm
Sampling rate: 1.7 L/min, 1-min interval [21] Polycyclic aromatic hydrocarbons 83-32-9 No threshold can be determined - - - - - - -
Pumped gravimetric method Sampling duration: 24 h [22] Trichloroethylene 79-01-6 4.3 × 10−7 μg/m
Model 2100 Mini- Partisol air sampler (Ruprecht & Patashnick Co.) coupled to a ChemPass model 34003 (unit risk) - - 37 mm diameter membrane (2 µm porosity) was used to collect particulate matters- [- 18- - -
] Tetrachloroethylene 127-18-4 0.25 mg/m3 (annual avg.) - - -
GRIMM environmental dust monitor, light scattering technology Sampling rate: 1.2 L/min, for 2 weeks (suitable for PM2.5 and PM1 also)- [- 17- -
] Ozone 10028-15-6 - 0.05 ppm (8 h avg.)

(0.100 mg/m		3) 0.1 ppm ≤240 µg/m3 (1 h) 0.1 mg/m3

(1 h avg.)		 100 μg/m3 (8 h) 0.1 ppm (1 h)

0.08 ppm (4 h)		 0.12 ppm (1 h)

0.08 ppm (8 h)
Minivol portable air sampler (Airmetrics, PAS 201) with pall flex membrane filter (47 mm) Filter conditioned in dry air for 48 h, sampling duration 5–7 h [23] Sulfur dioxide

(SO		2) 7446-09-5 - - 2 ppm (8 h avg.)

5 ppm (15 min)		 ≤50 µg/m3

≤1000 µg/m		3 (5 min) 0.15 mg/m3

(daily avg.)		 - 0.25 ppm (10 min)

0.2 ppm (1 h)
PM2.50.5 ppm (3 h)

0.14 ppm (24 h)

0.03 ppm (1 year)
PTFE filters (37-mm diameter, 2-μm porosity) Sampling rate: 1.8 L/min using a personal impactor, duration: 5 p.m. to 8 a.m. on weekdays and 24 h on weekends. Passive samplers and PM filters were stored in a freezer to keep them cool and avoid sunlight exposure [19] Relative Humidity (RH) -
Low volume sampling pump (model 224-PCXR8) with PEM impactor Every 5 min intervals- [<70% 24][- 2530–80%—summer; 30–55%—winter - - - -
] Radon (Rn) 10043-92-2 - - - 800 Bq/m3 (1 yr avg.) -
Airborne bacteria- - -
Burkard single stage impactor (Burkard Manufacturing Co. Ltd.) with an agar plate, followed by colony counting Sampling rate: 10 mL/min for 9 min, incubated at 35 °C in an oven for 2 days [21] PM2.5 - 25 μg/m3 (24 h avg.)

10 μg/m		3 (annual avg.) - - ≤40 µg/m3

≤100 µg/m (1 h)		 - - - 65 μg/m3 (24 h)
HCHO SKC formaldehyde monitoring kit: Colorimetric method Sample should be refrigerated and protected from sunlight and immediately sent to the air laboratory for analysis within 1 h [21] PM10 - 50 μg/m3 (24 h)

20 μg/m		3 (1 year) 150 μg/m3

(in office)		 - - 0.15 mg/m3

(24 h)		 - 90 μg/m3

(1 h avg.)		 150 μg/m3 (24 h)

50 μg/m		3 (1 year)

Health problems due to IAQ, which are more commonly respiratory-related diseases and allergies, have increased the importance of IAQ measuring techniques and associated tools. Therefore, device types and monitoring systems of different indoor air pollutants were extensively reviewed. Table 2 shows a summarized list of identified indoor pollutants and devices used for pollutant detection.

Table 2.

CO2, RH, temperature
Sample collection: Portable pump (Flec-FL. 1001 or Sibata) with 2,4-DNPH cartridge connected with ozone scrubber. Analysis: two stage thermo desorption followed by gas chromatography/mass spectroscopy
30 min ventilation of housing unit followed by 5 h of sealing. Samples were taken after that, 30 min each.
[
26
]
Radial diffusive samplers filled with 2,4-dinitrophenylhydrazine (2,4-DNPH)-coated Florisil (Radiello
®
code 165), analyzed by liquid chromatography with detection by UV absorption
Sampling duration: 2 weeks [19][20]
Diffusion sampler SKC UMEx100 based on chemosorbtion on 2,4-dintrophenyl htydrazine, analyzed by liquid chromatography Sampling duration: 1 week [14]
Air pull through 2,4-dinitrohydrazine (DNPH) coated silica gel cartridge (Supeleo LPDNPH S10) Sampling rate: 0.2 L/min for 40 min [23][27]
VOC Mass flow controllers (Model No. FC4104CV-G, Autoflow lnc.) trapped by Nutech Cryogenic Concentrator (Model 3550A), analyzed by Hewlett Packard Gas Chromatography (GC) (Model HP6890) using TO-14 method Sampling rate: 0.011 L/min for 8-h [21]
Diffusive samplers Exposure period of three days to two weeks [22]
Radial diffusive sampling onto carbograph 4 adsorbents (Radiello® code 145), analyzed by gas chromatography-mass spectrometry Sampling duration: 7 days [18][19]
Passive sampling (diffusion principle) with organic vapor monitors Middle of the room, height: 1.5 to 2 m [28]
Thermal desorption tube, analyzed by gas chromatograph/mass selective detector (GC/MSD) Sampling rate: 0.07∼0.1 L/min [16][27]
Proton transfer reaction mass spectrometer (PTR-MS) Sampling duration: Less than 5 min [29]
Tenax-TA tubes, analyzed by gas-chromatography with flame ionization detection (Varian, model 3700) & modified thermal desorption Sampling rate: 20 mL/min for 40 min [20][23]
Air pumped through a charcoal filter (Anasorb 747) Sampling rate: 250 mL/min for 4 h [14]
Air collected on adsorbent tubes and analyzed by gas chromatography-mass spectrometry Sampling rate: 100 mL/min for 100 min [30]
Organic vapor sampler, adsorbed on activated charcoal column, analyzed by gas chromatography-mass spectrometry Sampling duration: 8 h [17]
TBC RCS sampler (Biotest air samplers) following centrifugal impaction principle Sampling rate: 40 L/min for 4 min [23]
Rn CR-393 alpha track diffusion radon gas detectors Sampling duration: 3 months [31]
Alpha Guard Professional Radon Monitor Sampling duration: 1 week [15]
Passive measurements of Radon volumic activity by accumulating alpha radiation on 12 m cellulose nitrate film (Kodalpha dosimeter) Sampling duration: 2 months [18]
Passive dosimeters (Kodalpha LR 115 detectors) Sampling duration: 2 months, only in heating season [19]
Gamma Gamma radiometer of the Geiger-Muller type (Saphymo 6150 AD6) Sampling duration: 3–4 h [18]
Total Suspended Particulates & respirable suspended particulates (TSPs & RSPs) PVC filters (pore size 0.45 μm, diameter 37 mm, SKC, USA) Sampling rate: 2.5 L/min [27]
Lead (Pb) Airborne lead: mixed cellulose ester filter (pore size 0.8 μm, diameter 37 mm), analyzed with a Varian GTA100 model graphite furnace mounted on a Varian SpectrAA-880 model atomic absorption spectrophotometer based on NIOSH method 7105

Surface lead: collected with wet tissues based on NIOSH method 9100		 Sampling rate: 4 L/min [27]
Ammonia (NH3) Kitagawa precision gas detector tubes NA [14]
Airborne asbestos Open-faced mixed cellulose ester filter (37 mm diameter and 0.8 μm pore size) Sampling rate: 2.5 L/min [27]
Airborne micro-organism 25 mm nucleopore filter Pore size 0.4 nm, sampling rate 2 L/min for 4 h [14]
Mold & bacteria CAMNEA method Sampling rate: 4 h outside the window [14]
Bacterial aerosols Swirling liquid impingers Sampling rate: 12.5 L/min [17]

3. Conclusions and Future Scope

Most of the developed countries consider and follow IAQ regulations during the design and maintenance phase of building environments through appropriate measures. However, this scenario is not similar in developing or underdeveloped countries, where poor IAQ disproportionately affects children, women, and elderly persons [32]. Despite the severe impact of exposure to indoor air pollutants, there is still a lack of proper scientific research on IAQ in most developing and underdeveloped countries/regions. Analysis of peer-reviewed journals during this review indicated that primarily developed and a few developing countries are more interested in exploring IAQ in terms of the human health impact, whereas underdeveloped countries still lack IAQ-focused research. The pattern of indoor air pollutants in developing and underdeveloped countries and the consequences to health should be studied more, which can provide a baseline to determine more beneficial IAQ policies in these regions. Therefore, more research is needed in these regions to ensure healthy and sustainable building environments worldwide. Along with indoor pollution sources, the situation of IAQ is worse in some regions because of outdoor climatic conditions, such as high humidity, temperature, and dust intensity, such as in GCC countries. However, studies that have focused on the IAQ situation in GCC countries have mostly excluded detailed VOC evaluations.

The reviewed studies commonly examined some parameters, such as PM, volatile matters, carbon dioxide, and carbon monoxide; however, most have focused on selected VOCs. Although a few studies have analyzed VOCs in detail, most limited their studies to estimating TVOC, benzene, toluene, xylene, and ethylbenzene. Most studies have preferred to use gas chromatography-mass spectrometry to analyze VOCs, showing that it is the most popular detection method for VOCs. Analysis of carcinogenic air pollutants, such as radon, was rare. Additionally, few studies have clearly reported the building materials in walls or floors, whereas others did not mention the finishing type, furniture material, cleaning agent, household activities, which are highly critical elements for analyzing IAQ. Similarly, most studies focusing on commercial building’s IAQ have not specified the specific detail of the indoor materials that has the most impact on the air pollution. However, building structure and/or materials, surface finishes, and resident’s activity in general have been indicated as the major reasons for the elevated VOC concentration in the reviewed commercial buildings. Similarly, outside PM level and/or nearby construction process, tobacco smoke, presence of carpet, human movement have been identified for rise in indoor PM level where concrete additives has been indicated as the responsible element for higher indoor NH3 concentration. Moreover, inter-relation model or equation between pollutants concentration and pollution source inside building environment was not clearly presented in the reviewed studies. Therefore, this study recommends more studies focusing on detailed assessment of exposure concentration along with the identification of responsible sources in each type of building environment.

Of note, direct comparison of indoor air pollutant levels is difficult and not straightforward because evaluations have been conducted over different time periods, using different instruments and sampling techniques, and in different indoor environments. Thus, it is highly recommended that more detailed scientific studies be conducted by following standardized regulations, which will allow for an inter-comparison of IAQ from studies in the future to close the existing knowledge gaps regarding IAQ.

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