Extreme temperature episodes and mortality in Yakutsk, East Siberia

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1 Circumpolar Special Issue: Human Healh a he Ends of he Earh O R I G I N A L R E S E A R C H Exreme emperaure episodes and moraliy in Yakusk, Eas Siberia BA Revich 1, DA Shaposhnikov 1 Insiue of Forecasing, Russian Academy of Sciences, Moscow, Russia Environmenal Healh Laboraory, Insiue of Forecasing, Russian Academy of Sciences, Moscow, Russia Submied: 16 Ocober 009; Revised: 9 March 010; Published: 18 June 010 Revich BA, Shaposhnikov DA Exreme emperaure episodes and moraliy in Yakusk, Eas Siberia Rural and Remoe Healh 10: (Online), 010 Available from: hp:// A B S T R A C T Inroducion: Alhough he healh impacs of hea waves and, o a lesser exen, cold spells in big ciies in moderae climaes have been well documened, lile is known abou he same impacs in he circumpolar region. An epidemiological sudy in an Arcic own presens considerable difficulies for he saisician because of small populaion sizes. When daily moraliy couns are mosly 0, 1 or, hey are no normally disribued and do no fi he independence assumpion. The aim of his sudy was o ake hese difficulies ino accoun and assess he impacs of exreme emperaure evens on moraliy raes in Yakusk, a ciy wih a srongly coninenal climae, siuaed near he norh pole. Mehod: Long-erm disribuions of daily mean emperaures were analyzed for idenificaion of hea waves and cold spells during he sudy period of 1999 o 007. The auhors invesigaed daily moraliy from all non-accidenal causes, coronary hear disease and cerebrovascular causes among he age groups years and 65 years and over. Saisical analysis was in wo seps. Sep 1 involved Suden s -ess of samples, which consised of cumulaive moraliies during each hea wave. This provided a measure of he average healh effec of all idenified hea waves, and he same analysis was performed separaely for cold spells. A Sep, he BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 1

2 auhors compared he observed cumulaive moraliy during each individual emperaure wave wih expeced seasonal moraliy, using χ ess. Resuls: The analysis of he impacs of six hea waves and hree cold spells provided sufficien evidence ha cardiovascular and non-accidenal moraliies increased in Yakusk during boh hea waves and cold spells. The magniude of esablished healh effecs was approximaely he same for hea and cold. No significan differences were found beween he wo analyzed age groups in erms of relaive excess moraliy. Coronary hear disease moraliy increased more han wo-fold during some of he idenified emperaure waves, while non-accidenal moraliy increased by approximaely 50%. The ime lags beween he emperaure wave and observed increase in moraliy varied beween 8 and 14 days, which indicaed ha he healh effecs of emperaure exremes were delayed raher han immediae. The evidence obained of he effecs of emperaure waves on cerebrovascular moraliy was no conclusive. Addressing he mehodological implicaions of dealing wih small ciies, he auhors linked he sensiiviy of he applied saisical ess o arihmeic means and relaive sandard deviaions of daily deah couns, and o he duraion of emperaure waves. Conclusions: The proposed mehodology can be applied in oher medium-sized owns (populaions >00 000, approximaely); however, only large relaive increases in moraliy will be saisically significan. For example, relaive risks of less han.0 for coronary disease moraliy and 1.4 for non-accidenal moraliy are likely o be non-significan. Key words: climae warming, cold spell, coninenal climae, excess moraliy, hea wave, Yakusk Siberia. Inroducion There are a leas wo compelling reasons for focusing on public healh in norhern ciies wih a coninenal climae. Firs, greaer seasonal and diurnal variaions of emperaures are likely o have more pronounced healh consequences. Second, mos climae models predic ha circumpolar regions will experience greaer climaic changes han global averages. Recenly observed climaic changes in Siberia were noiceably greaer han he global rends. If he average global surface emperaure increased by 0.65 C over he las 50 years 1, hen in cenral inland pars of Siberia, he average annual emperaure increased by 1-3 C, and average winer emperaures increased by 3-5 C during he same period. This sudy concerned Yakusk in Russian Eas Siberia, locaed approximaely 4 (450 km) below he Arcic Circle. I is he capial of he Sakha (Yakuia) Republic, wih a populaion of according o he 00 Census. Yakusk is one of he coldes ciies on earh, wih January emperaures averaging 40.9 C. The coldes emperaures ever recorded ouside Anarcica occur in he basin of he Yana River o he norheas of Yakusk. However, July emperaures may exceed 30 C, placing he region among hose in he world wih he greaes seasonal emperaure variaions. The relaionship beween exreme emperaure evens and moraliy has been well documened in moderae climaes, where he greaes observed increases in oal moraliy ypically reach 60-85% 3-5, while he documened impacs of cold spells were generally more modes a 10-15% 6,7. Mos researchers agree ha cardiovasclular and respiraory deahs conribue mos o hese increases. Unforunaely, lile is known abou he same relaionship in circumpolar regions. One obvious reason is he absence of large ciies, which makes direc epidemiological sudies difficul. This sudy aemped o fill his gap and proposed a saisical framework o esimae variaions in moraliy ha can be applied o ciies where he oal daily moraliy is relaively low (4-6 deahs per day). The purpose of he curren sudy was o assess he impacs of exreme emperaure evens on moraliy raes in he seleced age- and cause-specific caegories of moraliy, and o idenify he mos vulnerable populaion groups. BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp://

3 The resuls of his sudy may be used o esablish populaion aler hresholds for when a specific climaic even (eiher hea wave or cold spell) is anicipaed. Because mos climae models predic widespread changes in exreme emperaures 1, he projeced increases in he frequency of exreme emperaure evens may be combined wih he resuls of he curren sudy o esimae he poenial direc effecs of climae change on moraliy in circumpolar regions. Mehods The sudy period spanned he 9 years beween 1999 and 007. A daabase of daily moraliy couns was developed and subdivided ino 6 caegories by age and cause of deah (Table 1). No disincion was made beween he sexes. The wo seleced age groups represened he able-bodied populaion: years and reired people aged 65 years and over; he number of deahs among hose aged under 30 years was oo small o analyze. The seleced causes of deah refleced our curren undersanding of climae-dependen causes: non-accidenal deahs and cardiovascular moraliy were analyzed. Based on absolue numbers of deahs, he wo mos significan causes were analyzed among cardiovascular deahs: ischemic (or coronary) hear disease (IHD) and cerebrovascular diseases (CVD). Mean daily emperaures for his sudy were calculaed on he basis of 3 hour air emperaure daa recorded by Yakusk weaher saion, which belongs o he nework of Federal Service for Hydromeeorology and Environmenal Monioring (Roshydrome). A hea wave was defined as a ime period during which he daily mean emperaure was above he 97h percenile of he hisoric disribuion of daily mean emperaures for 9 consecuive days or more, of which a leas 3 days had average daily emperaures above he 99h percenile. Similarly, a cold spell was defined as a period of a leas 9 consecuive days wih daily mean emperaures below he 3rd percenile, of which a leas 3 days had daily mean emperaures below he 1s percenile. Moraliy deseasonalizaion procedure Six hea waves and hree cold spells were idenified in Yakusk during he sudy period, which varied in lengh from 9 o 19 days. I was assumed ha he duraion of he moraliy response would be approximaely equal o he duraion of causaive emperaure wave. Thus, any slow changes in moraliy, which have ypical emporal scales of longer han approximaely 4 weeks, had o be filered ou prior o he analysis of he impacs of discree weaher evens. For his purpose, he observed daily moraliy M was reaed as he sum of a fas componen F, which responds o day-oday variaions of meeorological parameers, and a smooh funcion of ime Y, which models all longer-erm rends. The laer was modeled as a weighed moving window average of M, wih he widh of he moving window equal o he number of degrees of freedom (DF): = DF Y vim + i i= 0 All days of he sudy period, including emperaure wave days, were included in he esimaion of Y. The weighs v i in [1] were deermined as a compromise beween smoohness of funcion Y and goodness of fi: λ 1 λ + min goodness _ of _ fi smoohness where λ is a parameer, 0<λ <1, which ulimaely defines he degree of smoohing. By varying his parameer, i was easy o selec he desired amoun of smoohness of he Y funcion, as has been described 8. Goodness of fi is inversely N DF proporional o he sum of he residuals ( Y M ) ; = 1 while smoohness is inversely proporional o he sum of squares of he differences beween he firs derivaives, measured on each pair of subsequen days: [1] [] BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 3

4 N DF 1 ( Y + 1 Y + Y 1) =. In he model, DF=18 (half a year) was chosen o reflec general undersanding of he ime scale of he major possible confounders (long-erm changes in healh saus, seasonal variaions, winer epidemics of influenza and acue respiraory infecions). For example, here was no need o exclude epidemic periods from he analysis because poenial increases of moraliy due o epidemics were indirecly included in he algorihm for esimaing he expeced background moraliy. The desired amoun of smoohness of he Y funcion was limied o approximaely wo oscillaions per year. This seasonal smooh funcion, Y, was used as a proxy for he expeced (background) moraliy in subsequen calculaions of relaive increases in moraliy during hea waves and cold spells. As follows from he goodness-of-fi requiremen, he arihmeic mean of de-seasonalized moraliy F = M -Y over any exended periods of ime (more han 1 monh) should be very close o zero. This propery of he F funcion was used in he Tier 1 calculaions described below. Saisical mehods To obain evidence ha moraliy raes increase during emperaure waves, one has o analyze he ime series of daily moraliy raes obained over he years of he sudy. In a small ciy such as Yakusk, mos of he values in such a ime series are 0, 1 or. This limis he scope of applicaion of convenional saisical mehods based on dispersion analysis of independen samples, because: 1. The underlying disribuion ( paren populaion ) of daily moraliy raes does no follow a T- disribuion.. Moraliy raes, observed during consecuive days (eg during a discree weaher even) do no fi he independence assumpion. Because of his i was necessary o eiher recify hese problems or o use non-parameric ess, which do no require assumpions abou he underlying paren disribuions. In fac, boh were aemped, and evenually a wo-iered research sraegy was arrived a, which consised of he following sequence: Tier 1 - screening analysis of cumulaive impac of all six hea waves idenified during he sudy period; Tier - analysis of he healh impacs of individual hea waves. The same sraegy was hen applied o cold spells. Tier 1: Sudying cumulaive impac of emperaure waves The goal of Tier 1 analysis was o normalize he paren disribuion of daily moraliy raes and use dispersion analysis of independen samples drawn from his disribuion. Because all he hea waves were observed in differen years, heir healh effecs were assumed o be independen from each oher. In he Tier 1 analysis, he average, or cumulaive, healh impacs of all six hea waves were quanified, pooled ino one sample, and caegories of moraliy idenified where he impacs were saisically significan. Thus, Tier 1 was essenially a screening analysis, because i reduced he number of caegories of moraliy o be analyzed during he second ier (where he impacs of individual waves were sudied). Pearson s correlaions were calculaed beween same-day and nex-day moraliy raes o show if moraliy raes on consecuive days were no independen, and ha any samples of moraliy raes aken on consecuive days were no random. To check he long-erm disribuions of deseasonalized moraliy raes for normaliy, perceniles were calculaed, which corresponded o µ; µ±σ and µ±σ of he respecive paren disribuions. For all caegories of moraliy, such disribuions in Yakusk were srongly skewed o he righ (posiive skew). These were normalized by he use of an 11 day moving averaging procedure: 10 F + i = 0 = i F. Afer his ransformaion, F populaions 11 became normally disribued. The widh of he moving window (11 days) was chosen o reain an opimal amoun of BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 4

5 informaion abou he healh impacs of all emperaure waves. The average impac of all hea waves was characerized by he mean of he es sample of six F values, aken on he firs day of each wave. Indeed, if a hea wave begins on he day, hen F is he daily moraliy, averaged over he firs 11 days of he wave and expressed as he excess above he seasonal background. The individual observaions in he es sample were now independen, because each wave supplied a single observaion. The null hypohesis was formulaed as follows: he mean of he es sample X should be equal o he mean of paren disribuion (which is zero because we are dealing here wih de-seasonalized moraliy); and he sandard deviaion of he es sample should be equal o sandard deviaion of paren disribuion σ. Then, he value of Suden s -es is given by he following equaion: = X n σ, where n is he number of emperaure waves. The es samples did no differeniae beween he lenghs of individual waves, raher reaing hem as if hey each lased for 11 days. In realiy, however, all six hea waves had varying lenghs: 13, 1, 1, 19, 9 and 11 days (Table 3). The inenion during Tier 1 was o over-esimae, raher han o under-esimae he impac of all waves (o keep in more caegories for Tier ; his is a conservaive assumpion during screening). By seing he window widh o 11, he average daily impac of wave #5 may have been underesimaed, bu he average daily impacs of waves 1-4 were over-esimaed. When he window widh is shorer han he wave duraion, he healh effec is likely o be over-esimaed due o he harvesing effec: shor-erm displacemen of deahs makes one expec he maximum excess in deahs a he beginning of he wave, raher han a is end 9. Moreover, wave #4 appeared o be an oulier because i was much longer han he ohers. Wihou his wave, he average lengh of he remaining five hea waves was equal o 11. These observaions explain he choice of he moving window widh. Because he lieraure suggesed ha he impac of exreme emperaures on moraliy could be delayed by some days, he concep of ime lag was incorporaed ino he ess. For example, he sample of six F values, aken on he second day of each wave would characerize he average impac of all hea waves wih a ime lag of 1 day, and so on. The null hypohesis was esed wih alernaive ime lags, varying beween 0 and 0 days (as was suggesed in he Czech sudy 10 ) and idenified he ime lag, which maximized he probabiliy of rejecion of he null hypohesis. In all hese ess, however, i was implicily assumed ha he ime lags would be he same for all hea waves, which may no necessarily be so. This limiaion was embedded in he Tier 1 analysis bu i was relaxed a Tier. Tier : Sudying impacs of individual emperaure waves Because he small number of daily deahs in Yakusk precluded using parameric ess of independen samples for assessmen of impacs of individual emperaure waves, χ ess were used. This is a non-parameric es ha does no consider variance of daily moraliy raes. The observed healh impac of a emperaure wave was characerized by cumulaive moraliy, defined as he sum of daily deahs which occurred during his wave. Thus, i was posulaed ha he expeced response of moraliy should have exacly he same lengh as he duraion of he causaive emperaure wave. To calculae he expeced cumulaive moraliy, he seasonal smooh funcion of moraliy Y was muliplied by he duraion of a emperaure wave. Because he healh impacs of emperaure waves were no compared, χ ess wih DF=1 were used and, as a conservaive assumpion, also Yaes correcion for coninuiy, due o he small number of observaions. In he presen sudy, mos ess of cardiovascular causes involved cumulaive moraliies of less han 10; some were less han 5. A ime lag beween emperaure wave and moraliy wave of up o 0 days was allowed. BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 5

6 Resuls Table 1 provides basic saisics for he six caegories of moraliy sudied. As Table 1 shows, he greaer he average number of deahs per day, he smaller he relaive sandard deviaion of he corresponding disribuion. Table illusraes recenly observed rends in regional climae warming in Yakusk. During he las 50 years, winers in Yakusk have become milder, and exreme cold emperaures have risen from -50 º C o -45 º C, which grealy reduced he probabiliy of cold spells. During he same period, exreme ho emperaures in he summer increased only by 1 º C. As a recen aricle poins ou, no change in summer emperaures has been observed in he nearby Tobolsk region of Eas Siberia 11. Had hisoric perceniles been used in he definiion of cold waves, no cold waves would have been idenified during he sudy period (under he definiion provided in he Mehods secion). Because i was inended o reain as much informaion abou exreme emperaure evens as possible for furher analysis, a decision was made o use curren perceniles for he idenificaion of cold spells, insead of hisoric perceniles. As a resul, six hea waves and hree cold spells were idenified during he 9 year sudy period in Yakusk. Table 3 specifies he daes and duraions of hese emperaure waves. Table 4 summarizes he resuls of he Tier 1 analysis. The average impac of all hea waves was saisically significan a 95% in only one caegory of moraliy (IHD for ages years); he corresponding relaive moraliy risk was RR=1.4 [1.06; 1.79]. For all non-accidenal moraliy in he same age group, he impac of all hea waves was significan only a 93%. Afer excluding hea wave #3 from he es sample, he increase in his caegory became highly significan (=.47; p=0.015; RR=1.7 [1.13; 1.40]). This is he reason for also including his caegory of moraliy in he Tier analysis. The resuls of Table 4 for cold spells should be inerpreed wih cauion, because he number of analyzed cold spells was oo small for credible -saisics; hree waves are no sufficien. Indeed, here is sizeable probabiliy ha any hree randomly seleced F values are all posiive by chance. The proposed mehod canno give credible resuls if he number of emperaure waves analyzed is less han five. This requiremen is similar o he limis of applicabiliy of z- saisics or χ ess. Neverheless, for uniformiy of reamen of hea waves and cold spells, i was decided o use he resuls of Table 4 for cold spells as inpu for Tier analysis. Consequenly, only four caegories of moraliy for Tier analysis of cold spells remained, for which p was <0.05 (Table 4). Subsequen Tier hypohesis esing confirmed ha his was an appropriae choice. The resuls of he Tier analyses of he healh impacs of individual emperaure waves are summarized (Tables 5 & 6). These ables, unlike Table 4, canno conain confidence inervals of relaive risks, because he χ es reas cumulaive moraliy during each wave as a single value, and does no consider how i was disribued over he days of he wave. As Table 5 shows, only wo of he six hea waves significanly affeced moraliy from IHD (waves 1 & 5, wih corresponding RR.0 &.4), and hree waves significanly affeced moraliy from all non-accidenal causes (waves 1, 4 & 6, wih corresponding RR 1.5, 1.4 & 1.5). All four caegories of moraliy included in he analysis of cold spells showed saisically significan increases in deah raes during a leas wo of he hree cold spells ( a leas because of he conservaive assumpions embedded in he χ ess). The relaive magniudes of healh effecs in erms of excess cause-specific moraliy were approximaely he same for hea and cold. BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 6

7 Table 1: Overview of moraliy daabase in Yakusk: oal number of deahs in , mean daily raes and relaive sandard deviaions of daily deahs No. deahs Daily ICD-10 code Cause of deah Age group (years) No. deahs Daily Relaive Relaive SD mean SD mean I0 I5 Ischaemic hear disease I60 I69 Cerebrovascular diseases A00 R99 Non-accidenal deahs Raio of sandard deviaion o mean. Table : Comparison of hisorical and recen disribuions of daily mean emperaures in Yakusk Percenile Daily mean emperaure (ºC) Trend Hisoric Curren ( T) ( ) ( ) 1s rd h h Table 3: Daes and duraion of emperaure waves in Yakusk during he sudy period Wave Temperaure wave no. Hea Cold Daes Duraion (days) Daes Duraion (days) Table 4: Resuls of Tier 1 analysis: Suden s -ess of he null hypohesis concerning he average impacs of all deeced hea waves or cold spells Saisic Hea waves Cold spells years 65 years years 65 years Cause of deah IHD CVD All IHD CVD All IHD CVD All IHD CVD All Time lag p-value >.50.0 > RR % CI All, All non-accidenal causes; CVD, cerebrovascular diseases; IHD, ischemic hear disease; RR, relaive risk of moraliy. Time lag of he sronges link beween emperaure waves and moraliy; Excluding hea wave 3; Null hypohesis could be rejeced a he 95% level of significance (-ailed). BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 7

8 Table 5: Healh impacs of individual hea waves and ime lags, which maximize probabiliy of rejecion of he null hypohesis Wave no. Cause of deah/age (years) IHD/ All/ RR p Lag RR p Lag 1.0* * > * * > > * All, All non-accidenal causes; CVD, cerebrovascular diseases; IHD, ischemic hear disease; RR, relaive risk of moraliy. * RR significan (p<0.05). Table 6: Healh impacs of individual cold spells and ime lags, which maximize probabiliy of rejecion of he null hypohesis Cold spell Cause of deah/ age (years) no. IHD/ IHD/ 65 CVD/ 65 All/ 65 RR p Lag RR p Lag RR p Lag RR p Lag 1.6* < * < * * > * > * * < * All, All non-accidenal causes; CVD, cerebrovascular diseases; IHD, ischemic hear disease; RR, relaive risk of moraliy. *RR significan (p<0.05). Discussion In his aricle, he healh effecs of emperaure waves in a small ciy were esimaed. An iniial decision was made o purposefully limi he research o long hea waves only. Alhough he healh effecs of shor hea waves (<9 days) have been documened for big ciies, preliminary rials indicaed ha he mehods applied in he presen research failed o deec any healh effecs from shor waves in Yakusk. The size of he populaion under invesigaion is quie imporan, because i deermines he overall shape of he disribuion of daily deahs and he exen of naural variabiliy of daily deahs, measured by relaive sandard deviaion. For example, he effecs of emperaure waves on respiraory causes in Yakusk could no be deermined, he average number of daily deahs and associaed relaive sandard deviaions were µ=0.16; RSD=.5 in he age group years and µ=0.09; RSD=3.4 in he age group 65 years and over. Following his a wo-iered research sraegy was developed, where he firs ier was essenially a screening procedure o facor ou hose caegories of moraliy no as responsive o emperaure waves as ohers. The selecion crierion was saisical significance of relaive increase in moraliy, averaged for all deeced hea waves or cold spells. This did no mean ha he healh effecs of individual waves migh no be observed in hose caegories facored ou during Tier 1. A researcher dealing wih a muli-ciy sudy, or many sub-caegories of moraliy, wans o concenrae on caegories mos likely o display significan healh effecs. He/she may hen appraise he advanages of his sraegy. Boh iers have heir inrinsic mehodological value and may be applied independenly. Esimaion of he expeced healh impacs of an average emperaure wave may be imporan from a public healh sandpoin. Because climae models predic increases in frequency of emperaure waves over ime, i would be wise o base healh effec projecions on average measures. BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 8

9 Comparison of Tables 4-6 indicaes ha he Tier 1 es is somewha more sensiive han he Tier es (sensiiviy is he probabiliy ha a saisical es will be posiive for a rue saisic). Indeed, while RR=1.4 for IHD was saisically significan in Tier 1, RR=.0 for IHD was no significan in Tier (Table 6, cold wave 3). For all-cause moraliy, while RR=1.7 was significan in Tier 1, RR=1.3 was no significan in Tier (Table 5, hea wave ). Of course, such comparisons are valid only for he same caegory of moraliy. This resul is reasonable, because Tier 1 pooled he impac of six hea waves (or hree cold spells). As he number of waves included in Tier 1 analysis increases, increasing number of suble healh effecs will es posiive. In he presen sudy seing, he minimal (or criical) value of relaive risk ha is saisically significan a Tier 1 is provided by he following equaion: RSD RRc = 1+ c [3] n( w 1) where: c =1.96 is a criical value of a wo-ailed Suden s -es wih p=0.05 and DF= ; RSD is he relaive sandard deviaion of he paren disribuion of daily moraliy raes from Table 1; n is he number of hea waves (or cold spells) included in Tier 1 analysis; w is average duraion of a emperaure wave; in he sudy seing w=11. I is noeworhy ha Equaion [3] illusraes why shor hea waves should be excluded from analysis in small ciies. To give he reader he feeling of wha o expec from a Tier 1 analysis of a pooled sample of six hea waves in a small ciy like Yakusk, RR c =1.40 for IHD moraliy and RR c =1.18 for non-accidenal moraliy in he age group years. Smaller relaive risks will be facored ou during he Tier 1 analysis. Equaion [3] shows why relaive sandard deviaion of daily deah raes is so imporan for he sensiiviy of Tier 1 ess, which is no he case wih Tier analysis. Using Pearson s χ es, he criical value of relaive increase in deahs during an individual emperaure wave was given by he simple expression: RR = 1+ χ E [4a] c c / RR c is slighly higher using Yaes χ es: Eχc RRc = 1+ [4b] E where: χ c =3.84 is he criical value of he χ es wih DF=1 and p=0.05; E=wY is he expeced cumulaive moraliy during a given emperaure wave, which is he produc of he duraion of his wave w (measured in days) and he expeced daily moraliy, given by a seasonal smooh funcion of moraliy Y. The power of he χ es depends on boh he duraion of a hea wave w and he seasonal smooh Y, which differs from one year o anoher, even for he same calendar daes. As an esimae, average wave duraion w=11 may be used, and he daily mean raes from Table 1. Equaion [4b] hen gives RR c =.04 for IHD moraliy in he age group years and RR c =1.43 for all-cause non-accidenal moraliy in he same age group. How do hese RR c esimaes relae o he resuls of inernaional sudies? In a quick lieraure search of siespecific esimaes of relaive increases in cardiovascular moraliy observed during individual emperaure waves, only wo aricles were found ha repored RR close o or above.0. The RR=1.98 of cardiovascular moraliy among he age group 64 years and over was associaed wih he 1993 hea wave in Philadelphia 1, and RR=.39 for all-age BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 9

10 cardiovascular moraliy during a famous Chicago hea wave in More commonly repored values (>0 were surveyed) were much lower, usually in he range of RR= Therefore, he criical value RR c =.04 appears o be an oulier in he conex of inernaional sudies. As he ciies locaed in remoe and circumpolar areas are usually quie small, he following quesion has cenral imporance: wha is he minimal populaion size for which he impacs of individual emperaure waves can sill be saisically significan? This quesion can never be answered precisely because here are so many facors ha poenially influence he probabiliy of rejecion of he null hypohesis, and saisical mehods become ever more sophisicaed over ime. However, Equaion [4] can provide some guidance o researchers who sudy small populaions. In he ligh of he presen sudy s findings, i seems ha Yakusk is no far above his minimum size. I is hoped ha new sie-specific sudies in Arcic ciies may help us learn more abou healh responses under very harsh climaes and exreme weaher condiions. Conclusions Analysis of he impacs of six hea waves and hree cold spells provided convincing evidence ha cardiovascular and non-accidenal moraliies do increase in Yakusk during boh exreme hea and exreme cold weaher evens. More han wo-fold increases in IHD moraliy during wo hea waves were observed, while moraliy from all nonaccidenal causes increased by approximaely 50% during hree hea waves. All hese increases were observed only in he age group years. Analysis of individual impacs of he hree cold spells idenified during he sudy period, produced an even greaer number of saisically significan resuls in erms of increases in cause- and age-specific moraliy raes. The IHD moraliy increased more han wofold in boh age groups during wo cold spells, while CVD moraliy and moraliy from all non-accidenal causes increased only in he age group 65 years and over. Per one wave analyzed, cold spells produced more cause- and agespecific healh effecs han hea waves. Thus, cold spells seem o be more poen in erms of heir healh burden; however, hey are less frequen han hea waves. Esimaes showed ha almos all he increase in oal nonaccidenal moraliy during emperaure waves should be aribued o cardiovascular deahs. This conradics he conclusion of Huynen e al 7, who esimaed his proporion (for hea-relaed deahs) o be only 30%. Time lags of he mos significan healh responses varied beween 8 and 14 days, indicaing ha he healh impacs of boh hea waves and cold spells were posponed, raher han immediae. This finding agrees wih he resuls of a Czech sudy, where he lags of he sronges links beween cold spells and moraliy varied beween and 11 days 10. This conclusion could be imporan for public healh auhoriies, because i alers hem o expec elevaed moraliy and o plan for emergency healh proecion measures a leas for weeks afer a emperaure wave subsides. Acknowledgemens The auhors hank Vadim Ozheredov from he Space Research Insiue for consulaions on he saisical mehods, and Alexei Fedoov from Moscow Insiue of Physics and Technology for help wih processing he raw moraliy daabase. The auhors also hank Roman Ignaov from he Federal Service for Hydromeeorology and Environmenal Monioring, who helped hem access meeorological daa and commened on he qualiy and srucure of hese daa. The Insiue of Demography of Higher School of Economics provided moraliy daa for his research. The auhors are indebed o he anonymous reviewers whose commens helped hem o improve he original manuscrip. BA Revich, DA Shaposhnikov, 010. A licence o publish his maerial has been given o ARHEN hp:// 10

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