Dr. DERRICK CRUMP

The Building Research Establishment

STANDARDISATION OF AIR SAMPLING METHODS

Volatile organic compounds (VOCs) are a wide range of chemicals including; aliphatic and aromatic hydrocarbons, alcohols, aldehydes, ketones, esters, and halogenated compounds. They can occur in the outdoor air by release from natural sources, fuel combustion and industrial processes. The main focus of concern is the role they play in the formation of ozone and photochemical smog, and the increased risk of cancer due to exposure to benzene. VOCs are also present in the indoor air of non-industrial buildings, often in concentrations exceeding that found outdoors. They are then of concern because of the potential health effects (sensory, irritation and toxic) on the building's occupants. There is also concern about the possible deleterious effects on valuable materials such as electronic components in telephone exchanges and computers, and to works of art.

The outdoors is a source of VOCs in the indoor air but for most compounds, indoor sources such as; building materials and furnishings, consumer products, fuel combustion and tobacco smoke dominate. This results in the indoor concentration of VOCs being typically 10 times higher than outdoors. Since people spend the vast majority of their time indoors, exposure to air pollutants in the home is more problematic than exposure outdoors. Within Europe, the Construction Products Directive includes an Essential Requirement for Safety, Health and Hygiene that sets out a framework for achieving acceptable air quality in buildings. The interpretative document describes the use of ventilation and source control as means of achieving these objectives but currently there is no mandatory standard relating to air quality to assess compliance.

The largest study of VOCs in homes in the UK is the BRE Indoor Environment Study¹. This was a study of 174 homes in Avon, England, undertaken as part of the ALSPAC study which is being carried out by the University of Bristol. For 12 months, diffusive monitors were deployed on a monthly basis in selected homes to measure the concentrations of VOCs (including formaldehyde). Details of building characteristics and occupant activities were recorded and this enabled possible links with concentrations in air to be investigated. For example, it was found that painting activity strongly influenced the mean VOC concentration, formaldehyde was higher in newer homes, and benzene was higher in homes of smokers and those which have an integral garage ^1,2.

Other studies have been performed in the homes of 40 asthmatics in Southampton, and investigations have taken place in buildings where occupants were concerned about poor indoor air quality ^3,4. Injected damp proof courses, a liquid applied damp proof membrane product containing coal tar, ground contaminated with petrol, and poor quality chipboard flooring are all examples of sources which have an adverse impact on indoor air quality.

To measure the release of VOCs in indoor air, the methods of determination must be capable of detecting in the low ppb range. This means that in order to obtain sufficient sensitivity, it is necessary to trap the compounds of interest and then post-analyse using chromatographic techniques. Active methods, using air sampling pumps to draw air through adsorbents such as Tenax contained in a steel tube, and diffusive methods that do not require pumps are available with sufficient sensitivity and accuracy for many compounds.

Various environmental chambers are used in the laboratory to measure the release of VOCs from materials. The chambers range in volume from 'room size' to bench top apparatus. All types of chamber enclose the material to be tested in a controlled environment with a monitored supply of clean air. The VOC concentration in the air leaving the chamber is measured and used to calculate the rate of emission of VOCs from the source. Usually the chamber is controlled to provide temperature, humidity, air velocity and air exchange similar to that which may occur in a normally occupied room. The ratio of amount (surface area) of the source material to the volume of the chamber may also be selected to model a real room. A wide range of materials have been tested by laboratories in Europe and the USA and some labelling schemes for materials based on their emission of VOCs have been developed ^5,6.

In order to compare data between studies, to assess air concentrations against health based guidelines, and to quantify the amount of VOCs released by different sources, there is a need for reliable methods of measurement that can be widely applied. There are currently no British or International standards describing methods of measuring VOCs in air but there is considerable current activity with the European (CEN) and International (ISO) standards organisations to address this issue. The main initiatives are :

CEN TC 112 (Wood based board materials) WG5 (Formaldehyde) has produced a provisional standard (prEN717-1) for measuring the emission of formaldehyde from wood based materials such as chipboard using an environmental chamber.

CEN TC 264 (Air quality) WG7 (Building products - Determination of the emission of volatile organic compounds). This committee is preparing test methods for usage of environmental chambers and emission cells to quantify the rate of emission of VOCs from materials.

ISO TC 1446 (Air Quality) SC6 (Indoor air).
WG1 is providing guidance on the sampling strategy that should be applied for measuring pollutants in indoor air.
WG2 is producing methods for measuring formaldehyde and 3 standards will cover sampling strategy, an active (pumped) method, and a diffusive (passive) method. The test methods will utilise the reaction between formaldehyde and 2,4-DNPH to collect the formaldehyde from air, and analyses of samplers will be performed by HPLC.
WG3 is providing guidance on the measurement of VOCs. There will be a specified method for trapping VOCs on adsorbent tubes containing Tenax TA that can then be thermally desorbed to determine VOCs by GC. This standard will be adopted by CEN as the sampling and analytical method for measuring VOCs in chamber air. It intends to produce a standard on diffusive sampling in the future.
WG5 is providing general guidance on the measurement of VOCs in air (outdoor, indoor and workplace) and covers both pumped and diffusive methods.

It is recognised that there is a much wider range of compounds in indoor air that are of interest because of possible effects on the health of occupants. Examples include very volatile organic compounds such as vinyl chloride and the less volatile such as polyaromatic hydrocarbons (PAHs), and also inorganic gases such as nitrogen dioxide. For some compounds, e.g. PAHs, there are some relevant standardisation activities directed towards monitoring outdoor air. There is however, a need for a much wider range of standard methods than currently under preparation.

References

1. Berry RW et al, Indoor air quality in homes, The Building Research Establishment Indoor Environment Study, BRE Reports BR299 and BR300, CRC Ltd, Watford.
2. Crump DR, Indoor air pollution, in Hewitt C (Ed.), Air pollution in the UK, Royal Society of Chemistry Special Publication 210, 1 - 21, 1997.
3. Brown V, Crump DR and Mann H, The effect of measures to alleviate the symptoms of asthma on concentrations of VOCs and formaldehyde in UK homes, Proceedings of Indoor Air '96, Nagoya, Japan, 4, 69 - 74, 1996.
4. Brown V, Cockram A, Crump D and Mann H, The use of diffusive samplers to investigate occupant complaints about poor indoor air quality, Proceedings of Indoor Air '96, Nagoya, Japan, 2, 115 - 120, 1996
5. European Collaborative Action, Evaluation of VOC emission from building products, European Commission, Luxembourg, EUR 17334 EN, 1997.
6. Yu C and Crump D, A review of the emission of VOCs from polymeric materials used in buildings, Buildings and Environment, 33/6, 1998, 357-374.

ADDITIONAL NOTES

The Construction Products Directive framework has been fundamental in providing standards. The reason for developing pollution standards is to ensure that the air quality provides a healthy environment by controlling the concentration of pollutants. This is achieved by reducing the source, ventilating, filtering or by using absorbants to reduce the exposure limit.

Several documents already exist to help combat pollutants in indoor air, for example, the European Collaborative Action - Indoor Air Quality and its Impact on Man. Report No. 14 : 'Monitoring VOCs in Indoor Air' provides specific guidelines for sampling strategies and includes all the main considerations. This can perhaps be used as a good analogy for the museum environment.

ISO TC146 - Air Quality SC6
Working Group 1 - Sampling Strategy for Indoor Air Pollutants
Working Group 2 - Measurement of formaldehyde
Working Group 3 - Measurement of VOCs

Also there are a number of umbrella working groups, for example Working Group 5 : measuring VOCs in ambient, indoor and workplace air using active and passive sampling. Working Group 3 is concerned specifically with active sampling of VOC on Tenax TA and analysis by thermal desorption and GC. Working Group 2 is concerned with sampling strategies for formaldehyde - both passive and active (Use GMD badges for the passive sampling and DNPH impregnated silica powder cartridges for active sampling). The standards provide guidance on sorbent selection, preparation of calibration standards, analytical procedures etc.

Another useful document in preparation is prEN717-1 : Determination of HCOH release - part 1. Formaldehyde emission by the chamber method. The protocol is very specific and lists the test chamber conditions as 23°C, 45 % RH, 0.1-0.3 m/s air velocity, loading of 1m²/m³, air exchange must take place in 1 hour, sampling is performed twice daily and only after steady state conditions (or 24 h) has been achieved, this is defined as less than 5% change in a day.

Another useful standard is CEN TC246 WG7 : Building products - Determination of the emission of VOCs. Have commercially available emission cells. It involves the use of a test chamber and emission cells, and details the sampling and preparation procedures. Again it works at 23°C, 50 % RH, 0.1 - 0.3 m/s air velocity.