Natural ventilation needs supplementing

Astudy was undertaken between April and August 2003 to analyse the thermal comfort of two naturally ventilated ‘four-bed’ wards of a general hospital in The Midlands.

The study analysed an existing installation where the ventilation strategy was catered for by a “natural ventilation” solution. To assess comfort, Fanger’s Comfort Model1 has been considered, and reference made to the six main factors which should be quantified in order to assess human response to thermal environments in buildings. These are:

Air temperature.
Radiant temperature.
Air velocity.
Humidity.
The activity of the occupants.
The clothing worn by the occupants.

Case studies of two “naturally” ventilated bed areas have been considered. The area in Case Study 1 is north east facing and that in Case Study 2 is south west facing. The areas are sub-units of a 32-bed ward unit and have three centre-pivoted windows on one wall directly opposite the entrance.

Both internal temperature and relative humidity have been measured and recorded over the five-month period, focusing on the summer season in particular, to quantify the findings by analysis.

The monitoring process began in April 2003 by the placement of temperature and relative humidity loggers in the two bed areas, and continued until the end of August 2003. The loggers measured temperature and relative humidity in both rooms, sampling every 30 minutes/ 24 hours.

In the empirical field survey the staff and patients taking part were asked to assess their thermal sensation on a subjective scale ranging from “too cold” to “too hot”. This is generally known as the Comfort Vote. The results, while subjective, do give an important indication of the sensation of comfort for both staff and patients.

Impossible

Guidance ISO 77302 suggests that, due to individual differences, that it is impossible to specify a thermal environment that will satisfy everybody. Therefore we should expect to see a percentage of dissatisfied occupants.

In order to determine the most suitable thermal environment for wards in an acute hospital a fuller understanding of the activities, clothing etc. of both patients and staff was developed. As a development of this aspect, a questionnaire was produced to start a process to obtain information about the levels of satisfaction or dissatisfaction, particularly related to temperature parameters. This data would be extremely useful as a driver for firmer design criteria to be established.

A number of questionnaires were completed, and, coupled with the canvassed opinions resulted in a number of views which are summarised below.

Of those questioned, there was a marked difference between patients and staff views of their thermal environments (Comfort Vote). The majority of the staff voted hot at internal temperatures above 24°C, while on the other hand no patients voted hot at this value. Of the ten patients questioned, two voted warm, three slightly warm, four comfortable, one slightly cold and one cool.

This trend was noted throughout the specified period. Members of staff were dissatisfied with their environment for a good portion of that time, identifying that they were too warm or hot, particularly during June, July and August.

The patients on the other hand showed dissatisfaction when temperatures in the rooms did not fall at night but otherwise indicated satisfaction for most of the period.

Major effect

The thermal environment has a major effect on individuals’ perceived comfort levels and was used here as a criteria for this analysis. It was therefore necessary to consider and review both primary and secondary data available to determine temperature ranges for this scenario.

Using the clothing insulation values and the typical metabolic rates for staff, which are 0.5 clo and 2.0 met respectively, it can be established from the standards, for the probable level of activities of medical staff, particularly nurses, that the optimum operative temperature range is likely to be 21°C to 24°C.

From the observations, patients are comfortable in temperatures greater than those suggested for sedentary activities during the day. The optimum temperatures suggested by the standards, where clothing insulation is taken to be 0.3 clo and there is a metabolic rate of 1.0 met, is likely to be in the range of 23°C to 28°C during the day. The minimum of 23°C is based on the minimums given for sedentary activity. However, this may prove to be considered cool based on the responses.

Due to the patients’ responses over the specified period it is suggested that the upper limit of 28°C should be less at night. Based on their expression of dissatisfaction when temperatures were over 25°C, the upper limit at night might be 25°C. Relative humidity Health Technical Memorandum 20253 (Ventilation in Health Care Premises, Design Considerations) recommends the acceptable range of relative humidity (RH) for design purposes is 40% to 70% within healthcare premises. The recordings identified that RH dropped below 40% regularly, and down to 30% frequently, but rarely went above 70%.

It appears that this happened more in Case Study 1 than in Case Study 2. Internal relative humidity (RH) data was lost for the period of warmest weather for Case Study 1 (at the beginning of August). However, by interpolating from the remaining data it was determined that the RH in Case Study 2 was consistently higher than in Case Study 1 throughout the period, including the hottest spells during August, but only fractionally. That relative humidity falls to 30% on a number of occasions is a cause for concern.

It was noted that the internal temperature is affected by the number of sunshine hours in a day and over a number of days of continuous sunshine. At a particularly intense point (14 July), the area of Case Study 1 received direct sunlight through the three windows from about 4.00 am to around 10.00 am. As the rooms are at the top of the building, they experienced solar gains through the structure for most of the day. On the same day Case Study 2 experienced solar gains through the glazing from about 1.00 pm to around 8.30 pm. The Case Study 1 area, north east facing, reached similar internal temperatures as Case Study 2 during the day, but did cool down to a lower temperature at night. A number of factors would seem to cause this – wind direction being one – but solar gains also play an important role, the evidence indicates.

Wind direction and speed

For a period (14,15 and 16 July), the wind was predominantly northerly or north easterly at minimal speeds contributing to the night-time cooling in the Case Study 1 area. For a subsequent period (17 and 18 July), the wind was from the south west at a high speed (up to 5 m/s). This appears to have contributed to cooling the Case Study 2 area down to 20°C, which was overheating the previous days, but contributed little to the Case Study 1 area. Higher wind speeds appeared to accompany winds from the south and the south west, which had a greater influence on the Case Study 2 area than the winds from the north, which are generally at lower speeds, had on Case Study 1 through July. This trend is reflected throughout the specified period. In general the wind direction was predominantly south / south west and speeds of 1 to 3 m/s was experienced throughout.

Pollutants

The result suggests that for patients in these cases natural ventilation operates effectively from a single façade of the building for a large portion of the time. However, it also identified that with natural ventilation and uncontrolled air movements driven by wind difference in potential pressure, cross-ventilation occurred which creates air movement throughout the floor and picks up heat and pollutants. In healthcare premises, the later issue may carry the greater perceived risk.

It should be noted that contamination within the healthcare setting is primarily spread through contact rather than through the air. Where airborne, infection may be transmitted over short distances by large droplets, and over longer distances by droplet nuclei generated by coughing and sneezing.4 Literature also suggests that there is a clear pattern in which infection rates are lower where there is good air quality and patients are in single-bed rather than multi-bed rooms.5

Conclusion

The ventilation strategy has to do more than meet the air quality and comfort requirement for a building of today – as healthcare facilities have to last decades. High external temperatures were experienced in the UK this summer as well as in Europe. Record high temperatures were experienced in Europe during 2003, which resulted in high levels of mortality.6 In the majority of cases the hospitals had only naturally ventilated wards that were not able to deal with the ambient conditions and offered no relief from the heat wave for susceptible patients.

Since that time countries such as France have drawn up plans to help deal with future heat waves which include incorporating air conditioned / cooled zones within the hospital to which patients can be moved. The increase in ambient temperature will exacerbate the situation – there are predictions that the UK will have much hotter summers and increases in extreme temperature.

This study identified that, for a large period of time, natural ventilation was acceptable to patients, but much less so for staff, who found the conditions very uncomfortable during the summer months.

Natural ventilation should be utilised in the periods where it can provide environmentally acceptable conditions for both patients and staff, but it should be supplemented with a mechanical system that could provide a closer controlled temperature solution (with the provision of comfort cooling), and a better pressure maintenance regime (as forced flow). This could be used just within the summer periods, which would reduce the economic running costs, via the utilisation of the external environment to control the internal environment for the maximum period possible. This is the best sustainable solution.

The use of a mechanical system during the warmer periods would provide a healthcare setting that addresses the issues of comfort temperature, cross ventilation and air contamination, and global weather change patterns. It would provide a building better suited for its purpose of assisting people to recover.

References

1 Fanger P.O, Thermal Comfort, 1972 McGraw-Hill, New York.
2 ISO 7730, Moderate thermal environments, 1985 ISO Standard.
3 Hospital Technical Memorandum 2025, Ventilation in healthcare premises design considerations. 1994 NHS Estates.
4 Knight MD. Airborne transmission and pulmonary deposition of respiratory viruses – Airborne transmission and airborne infection. Enschede, Oosthoek Publishing Company 1973:175-183.
5 NHS Estates. The Environment for Care Conference – Prof. Roger Ulrich Keynote Address, September 2004.
6 Journal of the American Medical Association, Impact of Heat Wave on Mortality, June 2004.
7 CIBSE. Weather data with climate change scenarios. CIBSE TM34, October 2004.

Profile of authors

Michael Touhey has over the years managed the design process on a variety of projects both traditional and PFI. He is now employed by Catalyst Healthcare and is on secondment to Bovis Lend Lease North as a building services manager on the new Arena and Convention Centre in Liverpool. He began his career in the industry in 1973 as an apprentice electrician when he joined Duncan Watson, a London based contractor. In 1975 he joined Hoare Lea and Partners as a junior engineer.

Much of his experience, in the design of healthcare buildings, has been gained through working with a variety of consultants over a period of 30 years, including Ove Arup & Partners, YRM Engineers and JE Greatorex.

Michael Touhey completed an MSc in Building Services Management at Brunel University and received his degree in February 2004.

Carl McKenzie is a regional director with Faber Maunsell. His training was in mechanical building services engineering and environmental science, and he has established a successful track record in healthcare and laboratory related research projects.