A volatile organic compound (VOC) is one of a group of carbon based-compounds which readily evaporate into the air. Some of the best known VOCs include:

• Formaldehyde, found in a variety of products including board-backed wood, preservatives and carpet backing 
• Methylene chloride, found in paint strippers, aerosol spray paint and adhesive removers 
• Perchloroethylene, a solvent used widely in dry cleaning

There are various scientific definitions of a VOC. The one used in the European Union's Paint Directive is readily understandable – a VOC is an organic compound with a boiling point of 250°C or less at atmospheric pressure.

Outdoors, vehicle exhausts and emissions from burning wood are the main sources of VOCs. The main problem with VOCs outdoors is their reaction with nitrogen oxide emissions (from the same source) in sunlight to form a highly toxic mixture of pollutants called photochemical smog. This is sometimes observed in the urban environment as a brown haze over the city. Photochemical smog is most often seen in the summer months when sunlight levels are highest. One component of photochemical smog, ozone, is a particular health hazard. Other toxins, such as peroxyacetyl nitrate, have also been identified in photochemical smog.

Many VOCs found inside are synthetic in origin. These include:

• Board-based furniture and flooring boards. Board is a wood-based product that includes laminated board, chipboard (also known as particle board), hardboard, and medium-density fibreboard (MDF). These are made of wood bonded with a glue that contains formaldehyde
• Wood preservatives
• Adhesives
• Cleaning products
• Foam-backed new carpets and underlay
• Air fresheners
• Dry-cleaned clothes
• Paint and varnish. The solvent is added to paint to make it easier to apply. Paint may emit solvents when drying and some contain formaldehyde which continues to 'leak' into the air for many months once the paint is dry. Paint strippers are solvent-based and a source of VOCs
• Aerosols, including air fresheners
• Non-iron, easy-care or crease-resistant fabrics. These have been treated with a finish that contains formaldehyde

Organic solvents are the VOC component of many of these products. When exposed to the air, the solvent readily evaporates, adding to the VOC burden in the air. Often, the VOCs are emitted into the air from products like new carpet or board-based products over a period of months or even years – in a process known as out-gassing. Some VOCs do occur naturally. For instance, peeling an orange releases volatile compounds into the air, while a cut onion readily causes eye-watering because of the volatiles produced from its tissue.

Organic compounds vary greatly in their impact on human health. Many have no known health effects while others, like benzene (a known carcinogen) are definitely harmful. Many VOCs are irritants and can provoke symptoms among those with asthma, rhinitis and, especially, those with multiple chemical sensitivity. However, VOCs are not allergens in the way house dust mite, pet dander and pollen are.

Symptoms of exposure to VOCs may include:

• Throat, nose and eye irritation
• Headaches
• Liver, kidney and central nervous system damage
• Fatigue
• Breathlessness, wheezing

 Many VOCs cause cancer in animals but fewer are established as being human carcinogens. The following compounds are known or probably carcinogens:

• Formaldehyde
• Acetaldehyde
• Benzene
• Chloroform
• Naphthalene
• Paradichlorobenzene

 A special word of caution is needed on ozone which is not, in itself, a VOC but is formed by the reaction of VOCs with nitrogen oxides in the presence of sunlight. Exposure to ozone affects the lungs, causing inflammation and reducing lung function. Symptoms include:

• Pain, burning, and discomfort in the chest on breathing
• Chest tightness
• Shortness of breath
• Throat irritation
• Cough

Research has shown an increase in asthma attacks, hospital admissions and overall mortality on days when outside ozone levels are higher. Ozone exposure makes asthma symptoms worse and increases sensitivity to asthma triggers.

A number of VOCs in use in the workplace are controlled by the Control of Substances Hazardous to Health legislation. Outdoor pollution is generally controlled by environmental legislation which covers photochemical smog and ozone. For indoor VOCs, there are new laws from the European Union which lower levels of VOCs in paints, stains, and varnishes. The Volatile Organic Compounds in Paints, Varnishes, and Vehicle Refinishing Regulations (known for short as VOC 2010) means that the manufacture of non-compliant products stopped on 1st January 2010 and sale on 1st January 2011. Products are divided into different categories (matt, gloss, primer and so on) and different upper limits for VOCs laid down for each group. Within the group, limits differ for solvent and water-based products. This is to make sure that the product still does what it says on the tin.

Completely avoiding VOCs can be incredibly difficult. They are present in everything from our dry cleaning to furniture and the cosmetic products we trust. Opening windows and improving ventilation goes some way to reduce your exposure, however, if you live in a polluted area then you may simply be replacing one VOC for another - traffic pollution.

The most effective way to reduce your exposure to VOCs is through the use of an air purifier designed to remove them. For example the IQAir GCX VOC. Using effective air filtration is of critical importance to those with a VOC sensitivity or those working with VOCs where legal legislation may apply.