Indoor Air Quality (IAQ) and Indoor Air Pollution (IAP) – health impact of the most critical pollutants

Indoor Air Quality (IAQ) and Indoor Air Pollution (IAP) – health impact of the most critical pollutants

During the last two decades there has been increasing concern within the scientific community over the effects of indoor air quality (IAQ) on health. Changes in building design devised to improve energy efficiency have meant that modern offices are frequently more airtight than older structures.

Furthermore, advances in construction technology have caused a much greater use of synthetic building materials. Whilst these improvements have led to more comfortable buildings with lower running costs, they also provide indoor environments in which contaminants are readily produced and may build up to much higher concentrations than are found outside.

Indoor environment conditions contribute greatly to human wellbeing, as most people spend around 90% of their time indoors, mainly at home or in the workplace. According to the World Health Organization (WHO), indoor air pollution is responsible for the deaths of 3.8 million people annually.

IAQ can be generated inside buildings through occupants’ activities, such as use of electronic machines, use of consumer products, or emission from building materials. Harmful pollutants inside buildings include carbon monoxide (CO), volatile organic compounds (VOCs), particulate matter (PM), aerosol, biological pollutants, and others. Therefore, over the past decade, research on air quality control has begun to shift from outdoor to indoor environments.

Indoor Air Quality

IAQ normally is a complex mixture of particulate and various gaseous components. IAP compositions differ significantly depending on sources, emission rates, and ventilation conditions. For effective control of IAQ, therefore, it is necessary to determine the sources of air pollution. Moreover, the development of monitoring systems for the measurement of indoor pollutant concentrations as well as key strategies for control and enhancement of IAQ are considered essential. In this paper, we provide a comprehensive overview of the major IAP sources and IAQ-control strategies; we emphasize the sources, characteristics, and health effects of each IAP; we identify and discuss health issues and building-associated illnesses related to an IAQ decrease;

Pollutants with the strongest evidence for public health concern include particulate matter (PM), carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2) and Sulphur dioxide (SO2). Health problems can occur as a result of both short- and long-term exposure to these various pollutants. For some pollutants, there are no thresholds below which adverse effects do not occur. (Source: WHO).

Indoor Air Quality (IAQ) and Indoor Air Pollution (IAP)

According to the EPA’s definition, IAQ is the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants. IAP, meanwhile, refers to the existence of pollutants, such as volatile organic compounds (VOCs), particulate matter (PM), inorganic compounds, physical chemicals, and biological factors, all of which are at high concentrations in the indoor air of non-industrial buildings, and all of which can have negative impacts on the human body. In order to protect people from such pollutants, IAQ has emerged and been developed as a research field [9]. The main parameters for evaluation of IAQ include pollutant concentrations, thermal conditions (temperature, airflow, relative humidity), light, and noise.

It has been indicated that IAQ in buildings is significantly affected by three primary factors

  • Outdoor air quality, (ii) human activity in buildings, and (iii) building and construction materials, equipment, and furniture.
  • It is known that outdoor contaminant concentrations and building airtightness have a great influence on IAQ, due to the possibility of transportation of contaminants from outdoors to indoors

As outdoor pollutants’ concentrations increase, they are transported from outdoors to the indoor environment via ventilation. Hence, the correlation of outdoor air pollution with IAQ highly depends on the ventilation rate additionally to the lifetimes and mixing ratios of such pollutants.

In addition, equipment, such as computers, photocopy machines, printers, and other office machines, emit ozone (O3) and volatile compounds. Common building materials, such as poly (vinyl chloride) PVC floor covering, parquet, linoleum, rubber carpet, adhesive, lacquer, paint, sealant, and particle board, can shed toxic compounds (i.e., alkanes, aromatic compounds, 2-ethylhexanol, acetophenone, alkylated aromatic compounds, styrene, toluene, glycols, glycol esters, hexanol, ketones, esters, siloxane, and formaldehyde)

Here’s a brief outlook on the prominent indoor air pollutants, its source and its impact on occupant health their health effects, (Source WHO) and issues related to IAP-based illnesses, including sick building syndrome (SBS) and building-related illness (BRI).

Particulate Matter (PM 1.0, 2.5 and 10):


  • Through infusion of fresh air associated with air conditioning systems
  • Infiltration (through human and material movement inside airconditioned space)
  • Surface pollution

Health impact:

(PM) refers to inhalable particles, composed of sulphate, nitrates, ammonia, sodium chloride, black carbon, mineral dust or water. The health risks associated with particulate matter of less than 10 and 2.5 microns in diameter (PM10 and PM2.5) are especially well documented. PM is capable of penetrating deep into the lung and enter the bloodstream causing cardiovascular (ischemic heart disease), cerebrovascular (stroke) and respiratory impacts. Both long-term and short-term exposure to particulate matter is associated with morbidity and mortality from cardiovascular and respiratory diseases. Long-term exposure has been further linked to adverse perinatal outcomes and lung cancer.

 In 2013, it was classified as a cause of lung cancer by WHO’s International Agency for Research on Cancer (IARC). It is also the most widely used indicator for assessing the health effects of exposure to air pollution. 

Carbon Monoxide (CO):


  • Ambient air (vehicular traffic, gaseous effluent emanating from combustion)
  • Internal emissions from combustion activities (charcoal/wood use inside cafeteria/kitchen)

Health Impact:

Carbon monoxide is a colorless, odorless and tasteless toxic gas produced by the incomplete combustion of carbonaceous fuels such as wood, petrol, charcoal, natural gas, and kerosene.

Carbon monoxide diffuses across the lung tissues and into the bloodstream, making it difficult for the body’s cells to bind oxygen. This lack of oxygen damages tissues and cells. Exposure to carbon monoxide can cause difficulties breathing, exhaustion, dizziness, and other flu-like symptoms. Exposure to very high-levels of CO can lead to death.

Nitrogen Dioxide (No2): 


  • Ambient Air (coal, wood and oil smoke)
  • Space and water heaters
  • Appliances like stove or oven in kitchens

(Source: NCBI)

Health Impact:

Nitrogen dioxide is soluble in water, reddish-brown in color, and a strong oxidant. NO2 results from combustion processes such as those used for heating, transport and power generation.

Exposure to nitrogen dioxide can irritate airways and aggravate respiratory diseases. NO2 is an important ozone precursor, a pollutant closely linked to asthma and other respiratory conditions.

Ozone (O3): 


  • Ambient air (ground level discharge -photochemical reaction between oxygen-nitrogen oxides and VOCs)
  • Indoor sources (like photocopying air purifying disinfection devices etc.)

Health impact:

Ground-level ozone is a major component of smog. It is formed from photochemical reactions with pollutants such nitrogen oxides (NOx) emitted from vehicles, and industry. Due to the photochemical nature, the highest levels of ozone are seen during periods of sunny weather.

Exposure to excessive ozone can cause wheezing, asthma, allergy and coughing and in extreme cases scar in lung tissues.

Sulphur Dioxide (SO2):


  • Ambient air (as industrial air pollution- thermal power plant, refinery, metallurgical plants, paper and pulp and cement manufacturing)
  • Vented gasses from kitchens/food preparation units

Health Impact:

Sulphur dioxide (SO2) is the most common gas among the group of Sulphur oxides (SO2) present in the atmosphere.

Indoor SO2 levels are often lower than outdoor levels. SO2 emission indoors is usually small, owing to its reactivation, which can be easily absorbed by indoor surfaces. It is known that the hourly concentration of SO2 in buildings is often below 20 ppb

Human exposure to SO2, which can impair respiratory function, is only via inhalation.

Volatile organic compounds (VOCs):

Indoor source

  • Use of cleaning and personal care products;
  • VOCs, including formaldehyde, can be emitted from household products such as paints, paint strippers, wood preservatives, wax, pesticides, aerosol sprays, carpets, and many cleaning, disinfecting, cosmetic, and degreasing products

Health impact:

Volatile organic compounds (VOCs) are recognized as gases containing a variety of chemicals emitted from liquids or solids. Formaldehyde, a colorless gas with an acrid smell and which is released from many building materials, such as particleboard, plywood, and glues, is one of the most widespread VOCs.

VOC concentrations in indoor environments are at least 10 times higher than outdoors, regardless of the building location.

Because VOCs are organic chemicals that possess a low boiling point (Tb) and are easily volatilized even at room temperature, the WHO classified them into four groups: (I) Very volatile organic compounds (VVOCs) with Tb: 50–100 °C; (ii) volatile organic compounds (VOCs) with 100 °C < Tb < 240 °C; (iii) semi-volatile inorganic compounds (SVOCs) with 240 °C < Tb < 380 °C; and (iv) particulate organic matter (POM) with Tb > 380 °C.

Normally, exposure to VOCs released from consumer products is incurred via three main pathways: Inhalation, ingestion, or dermal contact. Most people are not seriously affected by short-term exposure to low concentrations of VOCs, but in cases of long-term exposure, some VOCs are considered to be harmful risks to human health, potentially causing cancer. As for SVOCs, transdermal uptake directly from air has a higher contribution compared with intake via inhalation.

The health effects of VOCs include increased rates of asthma, eye, nose and throat irritation, headaches, loss of coordination, nausea, damage to liver, kidney and central nervous system, and some are suspected or known to cause cancer.


  • Moisture/condensation (while the cooling or heating systems are non-operative)
  • Ductwork, air passages, constricted areas

Mold and bacterial growth can occur as a result of structural building faults, inadequate heating and insulation, or inadequate ventilation. These produce allergens and irritants that can cause asthma attacks among those allergic to mold. They also irritate the eyes, skin, nose, throat, and lungs of both mold-allergic and non-allergic people.

The sources and health effects of some common pollutants are listed in Table 1. Some of them can be present in both indoor and outdoor environments, while others originate from the outdoor environment. Generally, indoor air pollutants are able to be classified into organic, inorganic, biological, or radioactive.

Table 1

Common indoor pollutants and their effects on human health.

PollutantsSourcesHealth Impacts 
PMOutdoor environment, cooking, combustion activities (burning of candles, use of fireplaces, heaters, stoves, fireplaces and chimneys, cigarette smoking), cleaning activitiesPremature death in people with heart or lung disease, nonfatal heart attacks, irregular heartbeat, aggravated asthma, decreased lung function, increased respiratory symptoms 
VOCsPaints, stains, varnishes, solvents, pesticides, adhesives, wood preservatives, waxes, polishes, cleansers, lubricants, sealants, dyes, air fresheners, fuels, plastics, copy machines, printers, tobacco products, perfumes, dry-cleaned clothing, building materials and furnishings
  • – Eye, nose and throat irritation
  • – Headaches, loss of coordination and nausea
  • – Damage to liver, kidney and central nervous system
  • – Some organics can cause cancer
NO2Gas-fuelled cooking and heating appliances
  • – Enhanced asthmatic reactions
  • – Respiratory damage leading to respiratory symptoms
O3Outdoor sources, photocopying, air purifying, disinfecting devicesDNA damage, lung damage, asthma, decreased respiratory functions 
SO2Cooking stoves; fireplaces; outdoor air
  • – Impairment of respiratory function
  • – Asthma, chronic obstructive pulmonary disease (COPD), and cardiovascular diseases
CO2Cooking stoves; tobacco smoking; fireplaces; generators and other gasoline powered equipment; outdoor airFatigue, chest pain, impaired vision, reduced brain function 
Heavy metalsPb, Cd, Zn, Cu, Cr, As, Ni, Hg, Mn, Fe
Outdoor sources, fuel-consumption products, incense burning, smoking and building materials
  • – Cancers, brain damage
  • – Mutagenic and carcinogenic effects: respiratory illnesses, cardiovascular deaths
AerosolsTobacco smoke, building materials, consumer products, incense burning, cleaning and cookingCardiovascular diseases, respiratory diseases, allergies, lung cancer, irritation and discomfort 
Radon (Rn)Soil gas, building materials, and tap water
Outdoor air
Lung cancer 
  • – Termiticides, insecticides, rodenticides, fungicides, disinfectants and herbicides
  • – Building materials: carpet, textiles, and cushioned furniture
  • – Outdoor environment
Irritation to eye, nose and throat;
Damage to central nervous system and kidney;
Increased risk of cancer
Biological allergensHouse dust, pets, cockroaches, mold/dampness, pollens originating from animals, insects, mites, and plantsAsthma and allergies
Respiratory infections, sensitization, respiratory allergic diseases and wheezing
MicroorganismBacteria, viruses, and fungi are carried by people, animals, and soil and plantsFever, digestive problems, infectious diseases, chronic respiratory illness 


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Moreover, the IAQ guidelines of the WHO and USEPA are applied for the control of IAQ inside households, schools, hospitals, public buildings, and offices. In addition, each country will formulate specific standards or guidelines suitable to their own particular circumstances

Table 2

Indoor air quality guidelines for major indoor air pollutants.

PollutantsConcentration Levels (mg/m3)Exposure TimeOrganization
CO10015 minWHO
6030 min
301 h
108 h
291 hUSEPA
108 h
CO218001 hWHO
NO20.41 hWHO
0.1524 h
0.11 yearUSEPA
PM0.1524 hUSEPA
0.051 year
O30.15–0.21 hWHO
0.1–0.128 h
0.2351 hUSEPA
SO20.510 minWHO
0.351 h
0.36524 hUSEPA
0.081 year
Pb0.0005–0.0011 yearWHO
0.00153 monthsUSEPA
Xylene824 hWHO
Formaldehyde0.130 minWHO
Radon100 Bq/m31 yearWHO

The Effects of Indoor Air Pollution to Human Health

Building-Associated Illness

Over the past decades, various symptoms and illnesses have been linked to diminished IAQ in buildings and houses. Indoor exposure to inorganic, organic, physical, and biological contaminants, though often at low levels, is common, ubiquitous, and sustained. Therefore, the harmful effects of IAP on human health have always attracted great attention and concern. According to the WHO, building-associated illness refers to any illness caused by indoor environmental factors, which commonly are divided into two categories: Sick building syndrome (SBS) and building-related illness (BRI). Their associated symptoms are shown below.

The common symptoms of sick building syndrome (SBS) and building related illness (BRI).

Sick Building Syndrome (SBS)

Indoor Air Quality

Figure 2

The common symptoms of sick building syndrome (SBS) and building related illness (BRI).

SBS often refers to a group of symptoms that are linked to the physical environments of specific buildings. Acute health and comfort effects of SBS will appear when patients spend a certain amount or duration of time in a building, but they and their causes are difficult to clearly identify. These effects are either localized in particular areas or widespread throughout a building. It has been reported that symptoms tend to worsen as a function of the exposure time in buildings and can disappear as people spend more time away from the building. According to the WHO, SBS symptoms caused by IAP can be divided into four categories: (I) Mucous-membrane irritation: Eye, throat, and nose irritation; (ii) neurotoxic effects: Headaches, irritability, and fatigue; (iii) asthma and asthma-like symptoms: Chest tightness and wheezing; and (iv) skin irritation and dryness, gastrointestinal problems (i.e., diarrhea), and others. The International Labor Organization (ILO) reported that infants, the elderly, persons with chronic disease, and most urban dwellers of any age have higher health risks linked with IAP-associated SBS symptoms. A low ventilation rate, building dampness, and high room temperature also tend to increase the likelihood of SBS prevalence. Moreover, other risk factors, such as gender, atopy, and psychosocial factors, also have a significant influence on SBS symptom prevalence.

Building-Related Illness (BRI)

BRI describes illnesses and symptoms with an identified causative agent directly related to exposure to poor air quality in buildings. It is known that causative agents can be chemicals, such as formaldehyde, xylene, pesticides, and benzene, but biological agents are more widespread. In buildings, the typical sources for indoor emissions of biological contaminants are cooling towers, humidification systems, filters, drain pans, wet surfaces, and water-damaged building materials. BRI symptoms have been associated with the flu, including fever, chills, chest tightness, muscle aches, and cough. In addition, serious lung and respiratory problems are likely to occur. Common BRI illnesses include Legionnaires’ disease, hypersensitivity pneumonitis, and humidifier fever. It was reported that indoor environmental pollutants can cause BRI symptoms via four major mechanisms: (I) Immunologic, (ii) infectious, (iii) toxic, and (iv) irritant. An irritant effect is often BRI’s initial insult, but toxic, allergic, or infectious mechanisms can arise subsequently, depending on the pollutant type and individual susceptibility. Psychologic mechanisms are often not paid significant attention but are demonstrably likely to increase the overall morbidity of building-related diseases as well.

Four main factors have been linked to BRI, including: (I) Physical-environmental factors, (ii) chemical factors, (iii) biological factors, and (iv) psychosocial factors. As for the physical-environmental factors, BRI can be influenced by the temperature, humidity, lighting, air movement, and dust concentration. The chemical factors, meanwhile, include various pollutants released from human activities and products, such as carpets, paint, new furniture, smoking, cosmetics, asbestos, drapes, and insecticides. Finally, major biological factors associated with BRI are microorganisms as mentioned.

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