Airflow attenuation and bed net utilization: observations from Africa and Asia

Abstract

Background/methods: Qualitative studies suggest that bed nets affect the thermal comfort of users. To understand and reduce this discomfort the effect of bed nets on temperature, humidity, and airflow was measured in rural homes in Asia and Africa, as well as in an experimental wind tunnel. Two investigators with architectural training selected 60 houses in The Gambia, Tanzania, Philippines, and Thailand. Data-loggers were used to measure indoor temperatures in hourly intervals over a 12 months period. In a subgroup of 20 houses airflow, temperature and humidity were measured at five-minute intervals for one night from 21.00 to 6.00 hrs inside and outside of bed nets using sensors and omni-directional thermo-anemometers. An investigator set up a bed net with a mesh size of 220 holes per inch 2 in each study household and slept under the bed net to simulate a realistic environment. The attenuation of airflow caused by bed nets of different mesh sizes was also measured in an experimental wind tunnel.

Results: The highest indoor temperatures (49.0 C) were measured in The Gambia. During the hottest months of the year the mean temperature at night (9 pm) was between 33.1 C (The Gambia) and 26.2 C (Thailand). The bed net attenuated the airflow from a minimum of 27% (Philippines) to a maximum of 71% (The Gambia). Overall the bed nets reduced airflow compared to un-attenuated airflow from 9 to 4 cm sec-1 or 52% (p<0.001). In all sites, no statistically significant difference in temperature or humidity was detected between the inside and outside of the bed net. Wind tunnel experiments with 11 different mesh-sized bed nets showed an overall reduction in airflow of 64% (range 55 - 71%) compared to un-attenuated airflow. As expected, airflow decreased with increasing net mesh size. Nets with a mesh of 136 holes inch-2 reduced airflow by 55% (mean; range 51 - 73%). A denser net (200 holes inch-2) attenuated airflow by 59% (mean; range 56 - 74%).

Discussion: Despite concerted efforts to increase the uptake of this intervention in many areas uptake remains poor. Bed nets reduce airflow, but have no influence on temperature and humidity. The discomfort associated with bed nets is likely to be most intolerable during the hottest and most humid period of the year, which frequently coincides with the peak of malaria vector densities and the force of pathogen transmission.

Conclusions: These observations suggest thermal discomfort is a factor limiting bed net use and open a range of architectural possibilities to overcome this limitation.

Figure 1

The comfort zone. Airflow extends the comfort zone. The red arrows indicate the reduction in the comfort zone if airflow is decreased. At the upper edge of the comfort zone even a subtle reduction in airflow can result in a climate that is intolerable for most people (after Koenigsberger [6]).

Figure 2

Study locations.

Figure 3

Line drawings of the houses categories in which data were collected.

Figure 4

Set-up for measuring airflow under a bed net inside a home in The Gambia. One thermo-anemometer (TA) is placed (at 50 cm above ground level) under the bed net and a second thermo-anemometer is placed outside the net at the same height.

Figure 5

Experimental set up to measure the attenuation of airflow by nets in a wind tunnel. A: a fan is placed in a tube with a diameter of 100 mm to direct the airflow. The air flows through a second pipe in which a thermo-anemometer is placed to record the airflow. The airspeed (9, 14, 18, 24, 29, and 36 cm sec^-1) is measured at 6 different voltage settings. After recording airspeed in the absence of nets, the airspeed was measured after the airflow was attenuated by nets. B: Findings from wind tunnel experiments. The remaining airflow behind eleven bed nets at six different air speeds (9, 14, 18, 24, 29, and 36 cm sec^-1) was measured. The blue bars indicate the mean attenuation in airflow; the error bars indicate the range at the different airspeeds and the red line the average remaining airflow for all 11 bed nets (36%). (Sources of the bed nets: 1, 5, 6 - Philippines; 2 – Thailand; 3, 8 - The Gambia; 4, 7, 10 – Tanzania; 9, 11- Europe).

Figure 6

Indoor temperatures recorded over a 12-month period by HOBO data loggers. The hottest months of the year were identified for each site as the study period for the analysis. These were: The Gambia (February through June), Tanzania (January through April), the Philippines (April through December), and Thailand (April through October). The maximum temperature for each study house and each day during the study period was recorded and the mean daily maximum temperature for all houses in each site over the study period was calculated. To estimate the “bed time temperature” the daily temperature measurement closest to 21:00 hrs was selected for each study house and the mean temperature for all study houses over the study period was calculated.

Figure 7

Indoor climate during one night in a thatched mud house, Tanzania (3/03/2011; 20:30 hrs to 4/03/2011 05:30 hrs). The temperature drops from 29.5°C at 20:30 hrs to 27.5°C the following morning. Over the same time the relative humidity increases from 69% to 74%.

Figure 8

The effect of airflow on comfort. At 20:30 in the evening 63% of hypothetical people will feel uncomfortable despite a breeze (0.25 m/sec). The climate becomes more comfortable as the temperature drops during the night so that at 5:30 in the morning less than 30% of people will find the indoor climate uncomfortable. If we half the airflow from 0.25 to 0.125 m/sec the dissatisfaction increases and 50% of people will find it uncomfortable at 2:00 am to stay indoors. The differences in the percentage people dissatisfied between 0.25 and 0.125 m/sec and 0.125 and 0.06 m/sec are highly significant (Wilcoxon matched-pairs signed-ranks tests; p = 0.0001).