Smoke and mirrors

It seems obvious doesn’t it? Smoking pollutes the internal environment with a greater variety of chemicals than any other common activity – so ban it. A recent report by the British Medical Association says the effects of passive smoking have been underestimated. The survey of 5000 men in 18 towns shows a 50-60% increase in the risk of heart attacks through the exposure to other people’s smoke. The obvious, simplest and most effective way of reducing that risk is to remove the pollution source, in other words ban smoking.

However, smoking is still a legal activity and some people find it pleasurable. Many businesses claim their customers want the freedom to smoke and this is often backed up by compelling market research. On the other side of the argument those opposed to allowing smoking in public places react even to the faintest impression of tobacco odour or simply the sight of smoke. It is not just commercial pressures that affect business. Those same businesses also have a duty to ensure, so far as reasonably practical, the health and safety of their employees. Many businesses have therefore to deal with a number of competing pressures and must look for solutions that can, to some extent, satisfy both extremes.

The issue of indoor air quality goes beyond smoking. Over the last decade there has been a growing acceptance that the ‘soup’ of pollutants within the workplace environment is a contributory factor in sick building syndrome, increasing illness rates and absenteeism amongst staff. Environmental tobacco smoke is only one of a range of airborne contaminants to be considered.

These pollutants impact upon the occupant’s perception of a space. Recent studies show that perception of odour is affected by both humidity and temperature – the higher the total heat (enthalpy), the higher the dissatisfaction with a given amount of odour. An occupant’s perception of a space varies with time with the olfactory senses adapting quickly to different odours. Research into indoor air quality perception (comfort) has focused upon the first impression or unadapted response and consequently all design figures are based upon the objective of diluting odours to an imperceptible level.

In a strange way, when smoking was ubiquitous, it was a good indicator of the effectiveness of ventilation, as a smoky room was very obviously poorly ventilated. Today’s ‘no smoking’ rooms can still be poorly ventilated, with high contaminant concentrations from building materials, equipment and occupants, yet may appear acceptable.

The indoor air quality in a room is governed by four main factors:

• The method by which outdoors air is introduced to the room, (which defines ventilation effectiveness).
• The volume flow of outdoor air ventilation to the room.
• The concentration of contaminants in the outdoor air supply.
• The nature and rate of further airborne contaminants released within the room.

Environmental tobacco smoke is a mixture of some 4700 different substances – some of which can be smelled in minute concentrations. Its particulate content ranges from 0·01 microns through to 1·0 micron, well below the 2·5 micron threshold of the natural filtration mechanisms of the human body, so it progresses deep in the lungs and lodges in the alveoli.

Environmental tobacco smoke consists of two different types of smoke, mainstream smoke, which is that portion exhaled by the smoker, and side-stream smoke which comes directly from smouldering tobacco. This accounts for between 70% and 90% of total environmental tobacco smoke. In broad terms there are three ways of dealing with smoking in the workplace:

• Separate smokers and non-smokers.
• Provide local air cleaning devices.
• Enhance the ventilation system.

Smoker separation
This is perhaps the first step to controlling Environmental tobacco smoke within a building. Ideally the separation should be physical, with the space allocated to smokers maintained under negative pressure relative to the non-smoking area. In the absence of physical separation a mechanical ventilation system is required to control smoke movement within the room. Conventionally this is done by providing the supply air to the area occupied by the non-smokers and extracting from the zone occupied by the smokers. Whilst as Engineers we may see this as conceptually simple it is nevertheless important that the building manager or staff responsible for the building understand and follow the principles.

Local room-based solutions
BSRIA tests air cleaner equipment from manufacturers to grade their effectiveness against four benchmark indicators:

• Relative removal rate of particulate matter, also known as particle removal rate (prr).
• Air volume flow rate.
• Electrical power consumption.
• Relative acoustical performance.

These factors were measured in a test chamber constructed at the BSRIA and the measurements undertaken within guidelines set down by a standard testing specification, Specification S 22/99.

The air cleaners tested are devices with the function of removing particulate matter from the air. They are normally electrically operated and comprise a fan with a filtration assembly - typically bags, panels or electrostatic plates/wires. Their performance can be compared using the prr. While the clean and loaded test of prr provides a very good indicator for effectiveness of removing tobacco smoke from a room, other factors must be borne in mind:

• Price (unit and filter replacement/cleaning).
• Noise (high prr may mean a noisy machine).
• Aesthetics (does the equipment fit in with the environment or integrate with other services?).
• Reliability.

Over recent years a number of different products have come onto the market, that provide solutions to enable smoke to the treated locally. The most basic system is one where air passes through a room air-cleaner. Usually these will be electrostatic and in themselves can be quite efficient. However their room effectiveness may be quite low if their air turnover rate is insufficient.

A Japanese system, Tornex, uses the principle of smoker separation and air cleaning in combination with supply jets to capture and contain the smoke. Air is supplied into a smoking zone in such a way as to create a vortex or low pressure zone into which the smoke is drawn and from there either extracted and discharged or passed through a cleaner to remove the environmental tobacco smoke. The supply air also provides an air curtain to reduce smoke drift beyond the confines of the smoking zones. The concept is suitable as a retrofit solution and is easily portable. However care has to be taken that it is not located where there is a general mass movement of air across the space, typically found in large spaces such as airport terminal buildings as this will reduce the system’s effectiveness.

Another solution on the market is one developed by Halton Products and is designed to protect bar staff from smoke. This relies on a curtain of air directed up from a supply grille integrated the bar counter. The air curtain provides a barrier and a means of capturing the tobacco smoke, reducing exposure to bar staff. The system can be enhanced by warming the supply air to give it buoyancy.

Ventilation
The majority of ventilation and air conditioning systems are designed to dilute room contaminants, such as environmental tobacco smoke, to acceptable levels by mixing outdoor air with the room air. This results in an equal distribution of the concentrations of pollutants.

CIBSE guidance suggests an increase in unit outdoor air rates in line with the percentage of smokers amongst the room occupants. Rates of up to 36 litres/s per person are suggested and assume a mixing system with a ventilation effectiveness of 1·0.

Most mixing systems do not achieve perfect mixing implied by a ventilation effectiveness of 1·0. A more usual figure would be 0·8. To achieve a comparable condition the ventilation rate would be adjusted in accordance with the equation: Ve = Vd/Ev¹

Displacement ventilation is an alternative to mixed ventilation. It is now widely employed although sometimes poorly understood. The principle is simple: outdoor air being introduced at low level within a room and at 2-3°C below room temperature. The air spills out onto the floor creating a lake of relatively uncontaminated air. The depth of this lake will vary according to the thermal loads in the room because warm objects form convective plumes that will draw air from the lake.

Key to understanding this approach to ventilation to reduce contaminant exposure, is to recognise how the human body creates its own plume that can envelope the body and largely isolate the subject’s breathing zone from the surrounding room air, because the plume draws its source air from the lake. As a result the concentration of inhaled contaminant, like environmental tobacco smoke, is lower than that measured at an equivalent height within the room. The ventilation effectiveness can therefore be greater than 1·0 – typically 1·4 to 2·0, although even higher values are possible. This higher ventilation effectiveness means that a lower unit ventilation rate can be used in a displacement ventilation system than a mixing system to achieve the same level of comfort and exposure limitation.

Two points are often argued against the effectiveness of displacement ventilation. Where room surface temperatures create downward airflows, this must have the effect of reintroducing contaminants back into the occupied zone. While this is true, the contaminants enter the body of the room’s mix air – so providing the displacement plumes from the lake remain established, the air path to the breathing zone is largely unimpaired. The second point relates to the resilience of the plume particularly when moving. The plume is ineffective when walking but will quickly re-establish itself when movement ceases.

No guarantees
Smoking within buildings is a contentious issue particularly for the hospitality industry. The problems are often exacerbated by a lack of attention to the basics of indoor air quality control within many establishments. Even relatively simple measures such as extract fans can improve conditions for patrons and workers.

It is an inescapable fact that whatever systems are put in place there is no guarantee as to their effectiveness and inevitably pollutant exposure limits will be higher in places where smoking is permitted. Ultimately, it is a question of whether acceptable environmental tobacco smoke levels can be defined that so minimise the additional health risks as to be indiscernible from the inherent risks of living in an environment already often heavily polluted by transport and industry. It is a question that resides beyond the competence of engineers.

E-mail: nickcullen@rd.hoarelea.com