Volatile organic compounds or VOCs are molecules containing carbon which are predominantly in the gaseous state at room temperature. For example, most of the odorous molecules in flowers and scented products are VOCs . Because of their gaseous state and their small size, VOCs can interact with our respiratory system. Therefore, VOCs raise concerns regarding their influence on asthma and allergic disorders such as eczema or respiratory irritations. For instance, exposure to VOCs during pregnancy can lead to the development of eczema amongst newborns . As we spend about 90 % of our time indoors , it is particularly important to ensure that the chemical quality of indoor air is not causing any respiratory symptoms.
This article will present the possible sources of indoor VOCs, how VOCs impact on people with asthma and allergy and discuss the ability of air cleaners to reduce VOCs in the indoor environment.
Sources of indoor air VOCs
Amongst VOCs indoor air, several groups of compounds coexist. The most common ones are the following
|Type of VOCs||Molecules belonging to this type||Sources|
|BTEX||Benzene, toluene, ethylbenzene & xylene||Electronic devices, indoor smoking, cooking equipment(4), building materials(5)|
|Carbonyl Compounds||Formaldehyde, acetaldehyde, etc.||Building materials(5), textiles, furniture, combustion processes(6)|
|Terpenes||Limonene, alpha-pinene, Linalool, geraniol, dihydromyrcenol, etc.||Electronic devices,(7), essential oils, (8), air fresheners(9), fragranced products(1,9)|
|Alkanes||Nonane, hexane, methylcyclohexane, etc.||Electronic devices(7), adhesives(10), food cooking, consumer products(11)|
|PGE||Propylene, toluene, ethylbenzene & xylenes||Cleaning products, Paints(12)|
In other words, VOCs come from different sources:
- Building materials, which are the biggest source of indoor VOCs
- Electronic equipment
- Cleaning products containing PGEs
Building materials such as paints, solvents or varnishes have a key role in indoor air quality. Indeed, they release 40% of indoor air VOCs [5,12] amongst which are formaldehyde, acetaldehyde, toluene, acetone, propylene glycol and xylenes. As VOCs evaporate quite quickly, newly built homes tend to release more VOCs than older ones, especially during the first year after construction . Furniture releases roughly the same VOCs as building materials .
Electronic equipment releases VOCs too: alkanes, formaldehyde, acetaldehyde, terpenes and BTEX aromatic compounds are the most commonly found VOCs in electronics. VOCs in electronic equipment come from their plastic shell but also from the products they use, such as ink for printers. When used, most electronic devices heat up, promoting the evaporation of VOCs. This is particularly true for photocopiers, which emit more VOCs than most of electronic equipment .
Cooking and combustion processes are also associated with the release of VOCs. Some cooking processes have a greater impact than others. For instance, fryers and barbecues are amongst the biggest sources of VOCs in cooking. Barbecues release acetaldehyde and hexanal when used, up to 8 times the limiting values set by the US Environmental Protection Agency (EPA) .
Consumer products such as cleaning products, air fresheners, essential oils and self-care products do emit VOCs, especially when they are scented products . More specifically, they emit terpenes, a group of odorous molecules often found in flowers or fruits . Because of their chemical structure, terpenes are able to react with ozone to produce secondary organic aerosols, which can lead to elevated exposure to fine particles . Cleaning products also emit PGEs .
Impact of VOCs on asthma and allergic disorders
VOC exposure has been associated with eye and respiratory irritations, eczema as well as asthma . However, different effects of VOCs on human health have been reported regarding the type of molecules involved.
Exposition to PGEs such as propylene glycol has been correlated to the development of allergic symptoms, eczema, rhinitis, asthma and IgE sensitization in children under 8 years old .
Investigations on alkanes found no associations with dermatitis or allergic respiratory symptoms . One study linked N-undecane to asthma . Nevertheless, it is still important to respect the exposure limit values to such molecules and to ventilate kitchens where they can accumulate .
Terpenes undergo oxidation processes upon air exposure . Yet, if terpenes are not known to be allergenic, oxidized terpenes are associated with skin irritation and contact allergy [20,21]. Allergies to oxidized linalool and limonene are frequent and affect around 1% of the population [21,22,23].
Exposure to formaldehyde, a carbonyl compound, is known to induce skin allergic reactions and respiratory irritations [2,24]. Children whose mothers were exposed to formaldehyde during pregnancy are more prone to develop atopic eczema . It has also been linked to childhood asthma . Acetaldehyde does not have such effects but it causes bronchoconstriction amongst asthmatic persons  and it has been associated with an exacerbation of allergic airway inflammations .
BTEX exposure has been associated with asthma and rhinitis in adults . Children are also at risk as the presence of benzene in air has been linked to asthma and pulmonary infections [28,29]. Benzene, chlorobenzene and ethylbenzene trigger the production of antibodies when inhaled, leading to allergy sensitization .
In short, VOCs exposure is correlated to allergic reactions and asthma. Some VOCs such as formaldehyde or PGEs induce these reactions whereas others like acetaldehyde enhance respiratory symptoms in allergic or asthmatic persons. Therefore, it is critical to maintain an indoor air with low levels of VOCs for everyone.
Air treatment devices and their impact on indoor air VOCs
Different types of air purifiers exist and are commercially available nowadays. Some of them have an impact on VOCs while others don’t. Indeed, air fibrous filters (HEPA, ULPA, etc.) are designed to hold back dust particles and microorganisms but are not able to filter out VOCs . Used fibrous filters can even emit secondary VOCs if they are not replaced as they should be . Ionizers and electrostatic air purifiers, which use charges to precipitate or capture small particles are also inefficient against VOC pollution [33, 34,35].
Ozone generators can oxidize some VOCs such as terpenes. They are not an appropriate method to purify air as oxidized terpenes cause skin irritations . They cannot affect other VOCs  because they need to keep ozone concentrations under 0.00001% to avoid acute toxicity . Generally speaking, ozone generators should be avoided as their safety is not ensured.
Thermal plasma air purifiers
Non-thermal plasma air purifying systems can degrade up to 11% of certain VOCs like cyclohexene . To do so, they release ozone which, as previously mentioned, raises health concerns . Thermal plasmas are less prone to release ozone  and can be used to remove VOCs in industrial environments. They are not suited to treat indoor air as the VOCs concentrations in domestic indoor air are lower .
There are two air treatment technologies widely available which are designed to remove VOCs from indoor air.
- Materials which will capture the VOCs on their surface.
This phenomenon is called sorption and involves mostly weak physical bonds between the molecule and the sorptive material (adsorption). Sorption filtration using materials like activated carbon is an efficient option for the removal of VOCs [35,40] and it does not cause any secondary pollution when used. As adsorption does not degrade VOCs, sorption filters must be changed regularly. If possible, they should be sent back to the manufacturer as they can be regenerated .
- Photocatalytic oxidation (PCO)
The second option is to use photocatalytic oxidation (PCO), activated with UV light or fluorescent light, which degrade most of the VOCs in carbon dioxide CO2 and water [35,42]. PCO systems often use titanium dioxide as their catalyst . Depending on its design, the efficiency of PCO systems may differ . Furthermore, the chemical processes involved in PCO systems need several steps to go from VOCs to CO2. Partial oxidation can lead to the emission of secondary VOCs . Therefore, it is crucial to select systems which maximize the contact time between the air and the photocatalyst to obtain total oxidation of VOCs. To do so, these systems may use a pleated support for the photocatalyst  and low flow rates .
To conclude, there are a wide variety of VOCs coming from different sources. Building materials are the biggest source of indoor VOCs but they are not the only one . Electronic devices, especially photocopiers, and cleaning products containing PGEs (polyethylene glycol or glycol ethers) should not be used in rooms where children live [7, 12]. Indeed, the VOCs released by these sources lead to the development of respiratory and skin allergies as well as asthma . Some of them even enhance allergic reactions .
For everyone’s safety, air treatment systems can be used to lower the VOCs concentration. To act against VOCs, the air treatment devices need to have a sorptive filter such as activated carbon [35,40] or a photocatalytic oxidation system, often using titanium dioxide TiO2 .
About the author
Thanks to Dr Aliénor Dutheil de la Rochère for this insightful article.
allergies, asthma, VOCs, volatile organic compounds, indoor air, indoor environment, indoor air quality, allergen, filter, air filter, HEPA filter, particles, dust, particulates, air purifier, air cleaner, chemicals, formaldehyde, acetaldehyde, terpenes, alkanes, respiratory symptoms
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