Validation of the Wet Bulb Globe Temperature criteria (ISO 7243) and Predicted Heat Strain (ISO 7933) for older adults

The climate is changing and more and more extreme weather events, like heat
waves, are expected to occur in the near future. Models have shown that heat
waves will happen more intensely, more frequently and will be longer lasting.
These heat waves can lead to severe health effects and increase mortality. For
example, more than 70.000 people died in the summer of 2003 due to the heat
wave in Europe and most of these people were older adults. Older adults are
more vulnerable to the effects of high temperatures because of intrinsic
changes in the thermoregulatory system. For example, older adults start to
sweat at a higher core temperature and the thirst sensation is reduced compared
to younger individuals. Furthermore, older adults are often less physically fit
and have more illnesses and disabilities what makes them also more susceptible
to heat-related morbidity and mortality.

Individualized and timely advice on appropriate actions in thermal climate
stress may reduce morbidity and mortality among vulnerable populations like
older adults. However, current methods to assess thermal strain in hot
environments, like ISO 7243 and 7933, focus on normal, healthy adults. ISO
7243 is based on the Wet Bulb Globe Temperature (WBGT), which is a heat stress
index. The value of the WBGT represents the thermal environment an individual
is situated in. ISO 7243 provides reference values of the WBGT based on
metabolic rate, acclimatization status and clothing ensembles. When the
reference value of the WBGT is exceeded it is advised in ISO 7243 to reduce the
heat strain or to carry out a detailed analysis of the heat stress using ISO
7933. ISO 7933 is based on the Predicted Heat Strain (PHS) model and calculates
the increase in core temperature and sweat loss based on the thermal
environment. The maximum exposure time to the thermal environment can be
calculated with the strain criteria provided in ISO 7933. However, for older
adults these reference values of the WBGT may be different than for normal,
healthy adults. Also, the exposure time calculated with the PHS might be
different for older adults.

The aim of the current study is to investigate if the reference values of the
WBGT in ISO 7243 should be adjusted for older adults. Also, the calculated PHS
will be compared with the measured core temperature, sweat rate and exposure
time.

Participants visit the university on three separate occasions, once for
preliminary testing and twice for measurements in the climate chamber.
During the preliminary testing an electrocardiogram (ECG) analysis is executed
by the physician to detect any abnormalities in heart activity. Resting blood
pressure is measured to make sure the participant is not hypertensive and an
anamnesis questionnaire will be filled out. Based on these measurement the
physician decides if the individual can participate in this study. If the
decision is positive body height and weight is measured and the participant
does a YMCA cycle test to determine physical fitness.
At the end of the preliminary test participants are instructed to consume no
caffeine or alcohol and to avoid any strenuous exercise 12 hours preceding the
experimental trials. Participants are asked to consume 10ml/kg of body weight
of fluid 0-3 hours before both experiments to make sure they are euhydrated. In
addition, they were instructed to record their food and beverage intake during
the preceding 24 hours and asked to replicate this for both experimental
trials. Hydration status will be tested with a refractometer (Atago, Tokyo,
Japan) prior to both measurements.

In the experimental trials two classes of metabolic rate of ISO 7243 are
tested: class 0 (resting) and class 1 (low metabolic rate) as it is expected
most activities of older adults are in one of these two classes. In class 0 the
participant sits still during the measurements, in class 1 the participant
cycles on an ergometer at 10W which is considered to yield a metabolic rate of
between 160W and 200W. The experimental trials are on separate days in random
order, but at the same time of day. In both trials the participants wear
shorts, a t-shirt, underwear, socks and shoes. The climate chamber (b-Cat B.V.,
Tiel, The Netherlands) is set at 38°C and 28% (cycling trial) or 32% (sitting
trial) RH. When thermal balance is reached, determined by a stable core
temperature and heart rate for 10 minutes, relative humidity will be increased
every 10 minutes with 10%. The trials continue until core temperature increases
with a loss of thermal balance (0.1°C increase per 5 minutes for 15 minutes).
However, if this criteria is not met after spending 135 minutes in the climate
chamber the experiment will be ended, as well as when core temperature reaches
39.0°C.

During the time in the climate chamber the core temperature is measured using a
using a telemetry pill (BodyCap, Caen, France). Mean skin temperature is
assessed using iButtons (DS1922L, Maxim Integrated Products Inc, Sunnyvale,
CA, USA). Skin temperature is measured at eight different locations: Forehead,
right scapula, left upper chest, right arm in upper location, left arm in lower
location, left hand, right anterior thigh and left calf based on the ISO 9886
standard (16). Heart rate is measured with a wearable heart rate monitor (Polar
Electro, Kempele, Finland, RCX3 watch Polar Electro, Kempele, Finland) and
blood pressure is measured using a sphygmomanometer (Romed Holland, Wilnis,
the Netherlands). Local sweat rate is measured using the ventilated capsule
system. This system consist of a flowmeter (Omega Engineering, Stanford, CT,
USA) and temperature and relative humidity sensor (HygroVUE10, Campbell
Scientific, Logan, UT, USA). Heart rate is measured continuously and blood
pressure is determined every ten minutes before increasing humidity. Oxygen
uptake is measured with a metabolic card (COSMED, Quark CPET, Italy).
Participants are weighted on a weighing scale (Platform scale, SATEX 34 SA-1
250, Weegtechniek Holland BV, Zeewolde, The Netherlands) seminude before and at
the end of the measurement sessions so the amount of sweat loss can be
calculated. The participants are asked to rate their thermal sensation and
thermal comfort every ten minutes. Thermal sensation is assessed with a 9-point
scale (from -4=very cold to +4=very hot) and thermal comfort with a 5-point
scale (from 0=comfortable to +4=extremely uncomfortable)(17).

In the experimental trials two classes of metabolic rate of ISO 7243 are
tested: class 0 (resting) and class 1 (low metabolic rate) as it is expected
most activities of older adults are in one of these two classes. In class 0 the
participant sits still during the measurements, in class 1 the participant
cycles on an ergometer at 10W which is considered to yield a metabolic rate of
between 160W and 200W. The experimental trials are on separate days in random
order, but at the same time of day. In both trials the participants wear
shorts, a t-shirt, underwear, socks and shoes. The climate chamber (b-Cat B.V.,
Tiel, The Netherlands) is set at 38°C and 28% (cycling trial) or 32% (sitting
trial) RH. When thermal balance is reached, determined by a stable core
temperature and heart rate for 10 minutes, relative humidity will be increased
every 5 minutes with 1mmHg (2%). The trials continue until core temperature
increases with a loss of thermal balance (0.1°C increase per 5 minutes for 15
minutes). However, if this criteria is not met after spending 135 minutes in
the climate chamber the experiment will be ended, as well as when core
temperature reaches 39.0°C.

The participant can feel unwell due to the heat. During the experiment the
investigator will monitor the participant continuously by checking core
temperature and heart rate. Besides, blood pressure is measured every 10
minutes. Therefore the risk is small.