The meaning of indoor air quality
In the past there has been a degree of confusion over what indoor air quality really is. Currently there are two schools of thought – one looks at measuring carbon dioxide levels whereas the other favours the detection of volatile organic compounds. Stuart McSorley, Managing Director of Sontay tells us more.
Currently, there are no published UK standards or legislation for indoor air quality. The main standard governing occupational exposure to harmful substances is the ‘Control of Substances Hazardous to Health’ (COSHH). This sets out eight measures which employers or employees must take to maintain safe working practices, including risk assessment, control measures, monitoring and training. Exposure limits are specified in the HSE publication EH40/2005:Workplace Exposure Limits. Another useful publication is the ‘Air Quality Strategy for England, Scotland, Wales and Northern Ireland’ (DEFRA, July 2007).
In addition, indoor environmental design has been affected by targets on building energy efficiency, with the UK committed to reducing its carbon emissions by 20% by the year 2010 (based on 1990 levels) and encouraged by the Government Energy Efficient Best Programme and the Climate Change Levy. In part, these measures aim to minimise energy losses due to air leakage in buildings.
The measurement of indoor air quality has, conventionally, been used for comfort control and the prevention of Sick Building Syndrome (SBS). But what is meant by indoor air quality? Two schools of thought seem to exist. One, common in the USA, looks to measure carbon dioxide (CO2). The other which is more popular in Europe, looks to detect, but not accurately measure, volatile organic compounds (VOCs).
CO2 levels can be accurately measured by cost-effective non dispersive infra-red (NDIR) sensors, and is an excellent process variable to generate ventilation demand. Moreover, there is an equally important role to play in energy reduction, by minimising fresh air intake requirements. Fresh air needs heating or cooling and commonly contains a substantial amount of particulates and water vapour. This may require dehumidification and can cause indoor condensation and conditions where mould can grow or thrive, affecting not only how much energy is needed to condition the air but also potentially the fabric of the building.
Ventilation control using CO2 measurement is an enhanced method of control, allowing the control of ventilation on a zone-by-zone basis, similar to conventional temperature control. Currently, the majority of buildings use dampers which are set to provide a fixed amount of outside air – commonly for the life of a building – and the design, commissioning and maintenance must be carefully undertaken to ensure the duct distribution system can deliver the right amount of fresh air to each zone, based on intended occupancy. It can become a major undertaking to amend the air distribution systems to take account of office churn and changing occupancy requirements.
Indication of occupancy
Apart from actual CO2 measurements, sensors can also give indication of occupancy by registering increased levels of CO2 seen during occupancy compared to a lower, relatively stable ambient level during non-occupancy. This can be utilised to give ventilation-on-demand, rather than relying on the more commonly used fixed occupancy time profile, and result in even greater energy and cost savings. With CO2 ventilation control, demand tends to be based on how the building is operated rather than how it was originally designed, giving added flexibility to control. A simple system would see sensors placed in each zone so that air for each zone can be regulated based on actual occupancy and CO2 level. This is analogous to temperature control, in that a sensor in each space measures and regulates the delivery of warm or cool air to achieve setpoint. For these reasons, CO2 measurement is an ideal method for ventilation control in buildings.
It should be remembered that when using CO2 measurement for fresh air demand, it may be important to ensure that this demand can be overridden, for example where free cooling/night purge systems are in use and also during warm-up and cool-down periods where optimisers are used.
Indoor air quality, in the UK at least, is more commonly thought of in terms of VOCs. Indoor air quality sensors measure a broad range of contaminants. However, it’s important to remember, especially in light of the requirements of EH40/2005 that these sensors commonly do not measure levels of specific VOCs, but give an overall indication of contamination. Nor can they identify which VOC contaminant is present or its concentration.
Typical indoor environments in modern buildings have a large number of potential sources of organic chemicals, such as combustion sources, pesticides, building materials and finishes including paint, cleaning agents and solvents, and even plants and animals. Formaldehyde is a particularly common irritant and is used in the manufacture of carpets, insulation, textiles, paper products, cosmetics and certain plastics. It’s worth noting that, until recently, indoor air quality sensors were principally used to monitor levels of tobacco smoke in many public buildings, a function now obsolete due to legislation.
By the very nature of the majority of VOC contaminations, it may not be good practice (in offices at least) to control ventilation based on this type of sensor. VOCs are quite likely to come from a fixed, localised source, such as a carpet being cleaned or new paintwork, and simply increasing ventilation rate or fresh air makeup into recirculated air is unlikely – in the short term – to do more than provide temporary relief. It doesn’t deal with the source of the contamination, which is likely to be more important. For this reason, indoor air quality sensors may be much better suited for monitoring and alarm generation for the presence of VOCs, rather than for control of ventilation and fresh air demand generation.