Heat Related Indices for the Health Sector

Glenn McGregor (University of Auckland, NZ)Christina Koppe (German Weather Service, Germany)

Sari Kovats (London School of Hygiene and Tropical Medicine, UK)

The Challenge of a Climate Index for the Health Sector

• A heat health index attempts to describe the complex interactions between humans and their surrounding environment (atmospheric and built environments)

• Many factors confound/modify the relationship between heat and health (health status, age etc)

• The heat health relationship may not be stationary on a range of time scales (intra-seasonal to inter-annual to decadal; the climate (heat) health relationship is dynamic

• Biometeorologists generally use “biomet” indices while Epidemiologists tend to use standard climate descriptors such as Tmax, Tmin and RH to represent heat stress

gerd.jendritzky@dwd.de DWD 2003

Human Biometeorology

THE ATMOSPHERIC ENVIRONMENT

heat

temperature

radiation air pollution

light liquids

Source: WMO/WHO Guidance Notes on HHWS

Temp Condition

44 heat stroke, brain damage

41 Fever, very heavy exercise

38 normal resting condition

36 shivering

35 severe shivering

33 reduced consciousness ventricle fibrillation

3114

31 “death”14 lowest measured temperature with full recovery

Heat Cramps Heat Exhaustion Heat Stroke

Core Temperature

2% thirst

4% + dry mouth

6% + increased heart rate + increased body temperature

8% + swollen tongue, difficult speech, reduced mental and physical performance

12% recovery only after IV or rectal fluid administration

14% fast temperature increase, death

Heat Tolerance & Water Balance Dehydration(%) and Effects

Definitions of Heat(Thermal Assessment Procedures)

• Simplified Biometeorological Indices (temp and humidity)– Heat Index– Humidex– Net Effective Temperature– Apparent Temperature– Wet Bulb Globe Temperature

• Heat Budget Models

– COMFA (Brown and Gillespie, 1986), Effective Temperature (Gagge et al., 1971) Predicted Mean Vote (PMV) (Fanger, 1972), Klima-Michel (Jendritzky and Nubler, 1981), Physiologically Equivalent Temperature (PET) (Hoppe, 1999), MEMI (Hoppe 1984), MEMEX (Blazejczyk, 1994), RAYMAN (Matzarakis et al., 2009), ENVI-Met (Bruse 2004) and SOLWEIG (Lindberg et al. 2008)

– UNIVERSAL THERMAL COMFORT INDEX (UTCI)

0SeTa,Qve,Qve,QvTmrt,*QvTa,QWM ReSWLH

Heat Index• Combines air temperature and relative humidity to determine an

apparent temperature — how hot it actually feels. • HI is widely used in the USA and is effective when the

temperature is greater than 80ºF (26ºC) and relative humidity is at least 40%.

• Heat Index(HI) =

-42.379 + 2.04901523(Tf) + 10.14333127(RH) - 0.22475541(Tf)(RH) -6.83783x10**(-3)*(Tf**(2)) - 5.481717x10**(-2)*(RH**(2)) + 1.22874x10**(-3)*(Tf**(2))*(RH) + 8.5282x10**(-4)*(Tf)*(RH**(2)) -1.99x10**(-6)*(Tf**(2))*(RH**(2))

• Where Tf = air temperature in degrees Fahrenheit, RH= relative humidity expressed as a whole number. (for conversion: Tc = (Tf – 32) * 5 / 9 )

Humidex• The humidex is a Canadian innovation• The humidex combines the temperature and humidity into one

number to reflect the perceived temperature. • Humidex = (air temperature) + h

h = (0.5555)*(e - 10.0);e = 6.11 * exp(5417.7530 * ((1/273.16) - (1/dewpoint)))

• The range of humidex values and the associated degree of comfort is given below:

Less than 29     : No discomfort 30 to 39             : Some discomfort 40 to 45             : Great discomfort; avoid exertion Above 45           : Dangerous Above 54           : Heat stroke imminent

Net Effective Temperature (NET)

• The net effective temperature (NET) takes into account the effect of air temperature, wind speed and relative humidity.

• NET = 37 - (37-T)/(0.68 - 0.0014*RH + 1/(1.76+1.4*v**0.75) ) - 0.29*T*(1-0.01*RH) • T= air temperature (°C), v = wind speed (m/s), and RH = relative

humidity (%). • People will feel cold or hot when the value of NET is

equivalent to the lowest or highest of 2.5% of all values. • In Hong Kong, a Cold (or Very Hot) Weather Warning is issued

when the NET is forecast to be lower (or higher) than the 2.5th percentile (97.5th percentile). This procedure is also used for example in Portugal.

Wet-Bulb Globe Temperature• The Wet Bulb Globe Temperature (WBGT) combines temperature and

humidity into a single number• The WBGT is measured by a simple three-temperature element

device. • First temperature, (Tg), is measured by the black globe

thermometer• Second thermometer measures the natural wet-bulb temperature

(Tnwb). • Third temperature is (shaded) air temperature (Ta)

• The three elements Tg, Tnwb, and Ta are combined into a weighted average to produce the WBGT. • WBGT = 0.7 × Tnwb + 0.2 × Tg + 0.1 × Ta

• Australian Bureau of Meteorology uses an approximation based on standard measurements of temperature and humidity to calculate an estimate of the WBGT The simplified formula is: • WBGT = 0.567 × Ta + 0.393 × e + 3.94• where: Ta = Air temperature (°C), e water vapour pressure (hPa).

Apparent Temperature• The Apparent Temperature (AT) is defined as the temperature, at the

reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity.

• Basically the AT is an adjustment to the ambient temperature (T) based on the level of humidity

• The Heat Index is a simple hot weather version of the AT• The formula for the AT used by the Australian Bureau of Meteorology

is an approximation of the value provided by a mathematical model of the human heat balance

• AT = Ta + 0.348×e − 0.70×ws + 0.70×Q/(ws + 10) − 4.25 (includes radiation)

• AT = Ta + 0.33×e − 0.70×ws − 4.00 (no radiation)• where: Ta = Dry bulb temperature (°C), E = Water vapour pressure

(hPa), ws = Wind speed (m/s) at an elevation of 10 meters, Q = Net radiation absorbed per unit area of body surface (W/m2)

• It should be noted that when using the term AT one must keep in mind that there are three different versions of AT

Appa

rent

Tem

pera

ture

(oC)

Heat budget models• The human heat budget can be written as:

• M Metabolic rate (activity)• W Mechanical power (kind of activity)• S Storage (change in heat content of the body)• Skin:• QH Turbulent flux of sensible heat • Q* Radiation budget • QL Turbulent flux of latent heat (diffusion water vapour)

• QSW Turbulent flux of latent heat (sweat evaporation) • Respiration:• QRe Respiratory heat flux (sensible and latent)

0SeTa,Qve,Qve,QvTmrt,*QvTa,QWM ReSWLH

• Comfort Formula (COMFA) (Brown and Gillespie, 1986), • Effective Temperature (Gagge et al., 1971)• Predicted Mean Vote (PMV) (Fanger, 1972),• Klima-Michel (Jendritzky and Nubler, 1981),• Physiologically Equivalent Temperature (PET) (Hoppe, 1999),• Munich Energy-Balance Model for Individuals (MEMI) (Hoppe

1984)• Man-Environment Heat Exchange Model (MEMEX)

(Blazejczyk, 1994)• Radiation and Human Bioclimate Model (RayMan) (Matzarakis

et al., 2009)• Environmental Meteorology Model (ENVI-Met) (Bruse, 2004)• Solar and Longwave Environmental Irradiance Geometry

Model (SOLWEIG) (Lindberg et al. 2008) • UNIVERSAL THERMAL COMFORT INDEX (UTCI)

UTCI• Developed by a group of biometeorologists – EU

Cooperation in Science and Technology (COST)• A state of the art comfort index that capitalized on 30 – 40

years of human heat budget modeling and has wide (“universal”) acceptance amongst researchers and end users.

• UTCI – a multi-node, multi-layered thermo-physiological representation of

the human body with spatial subdivisions– a state of the art adaptive clothing model– model output in the form of an equivalent temperature which is

related to a strain index that represents the covariant behavior of metabolic rate, core temperature, skin wettedness, blood flow and sweat rate

Thresholds (an exceedence temperature or biomet index)

as Indices

Relationship between cold, heat and mortality

Index of Heat

Mo

rtal

ity

Low oC

High oC

Keatinge et al., 2000: British Med. J.

Thresholds (Apparent Temperature oC)

E ff ect o f su m m er tem peratu re on to ta l m orta lity , city th resh o ld an d % variation o f m ortality fo r 1 ° C in crease in m ax im u m apparen t tem peratu re( B igger i et a l, 2 0 07 , subm itted )

P H EW E P R O J ECT R ESU LTS :

95

100

105

110

115

120

125

130

-3 -2 -1 0 1 2 3

thermal load category (PMV)

rela

tive

mo

rtal

ity

(% E

V) The Netherlands

Baden-Württemberg

Lisbon

London

Budapest

Madrid

Heat Load Mortality Relationship: Europe

Thresholds are Used to Call Heat Health Warnings

05

10

15

% o

f da

ys

0.8

1.0

1.2

1.4

1.6

1.8

R

R

10 15 20 25 30 35Temperature

London

Epidemiologic association

[Two-day mean of tmax, after adjusting for potential confounders]Source: Sari Kovats

05

10

15

% o

f da

ys

0.8

1.0

1.2

1.4

1.6

1.8

R

R

10 15 20 25 30 35Temperature

London

Action threshold (“trigger”)Trigger for Heat Plan = 320C (max)

Threshold = (forecast) temperature above which action is taken to abate the adverse impact of heat Source: Sari Kovats

When to Call a

Warning?

• To prevent as much heat-related mortality as possible a low threshold would be defined (e.g. Threshold T1).

– In this case, a lower threshold may be chosen, and while the amount of lives that could be saved could be very large (amount a), the cost would be high and warning fatigue may set in.

• Or define only very severe situations as “sufficiently hazardous” (Threshold T3).– The aim of such a system is to prevent only the mortality peaks during very extreme conditions. – As such conditions are very rare and might occur only once in several years, the total number of

lives saved with such a system is smaller (amount c), although the costs to run any mitigation would be less.

heat indicator

exce

ss m

orta

lity

T1 T2 T3

heat indicator

cum

mul

ated

exc

ess

mor

talit

y T1 T2 T3 c

b

a

Source: WMO/WHO Guidance Notes on HHWS

Simplicity or Complexity?

Simple vs Complex Biomet Indices

Source: Christina Koppe

Percentiles as a “Universal” Indicator ?

• Japan• 80 – 85th percentile of Tmax =

optimum temperature• Optimum temperature is the value

where there is no apparent health effect

OT

Source: Honda et al., 2007

Is there a parsimonious “ideal” index for heat related health

problems?