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13 InstituteforSocialandEnvironmentalTransition www.i7s7e7t.org can adapt. Any heat index values approaching a human body skin temperature of 35°C core temperature of 37°C, particularly for sustained periods, can lead to negative health impacts. As seeninFigure2andTable1,evenahealthypersonengagedinmoderatephysicalactivitycanbegin experiencingfatigue,irritabilityanddifficultyconcentratingatheatindexvaluesrangingbetween26 to 32°C. Outdoor workers engaged in construction, fishing, farming or street vendors fall into this category. These heat index ranges are applicable to unhealthy individuals not engaged in physical labor–thosewithcardiac,respiratory,chronicillnesses,malnutrition,diabetesorotherconditions. Thesepeople,aswellasolderindividuals,childrenandpregnantwomenhavelowerheattolerance. Asheatindextemperaturesrise,healthyworkersengagedinlightactivity,suchasworkingatadesk, are negatively impacted. Indoor workers doing physical labor – manufacturing, sewing in factories, cleaning and heavy housework – will begin suffering negative health effects at the same ranges as outdoorworkers.Allofthesefactorshavetobeconsideredwhenassessingpotentialclimatechange impactsonheatstress. HotDays Between1970and2011,therewereanaverageof186274daysperyearinwhichtheheatindex wasequaltoorgreaterthantheMOH’srecommendationsof34°C30°Cforlightheavylabor.On average,theheatindexis4°CwarmerthantheambienttemperatureFigure6.Interestingly,while the number of days per year in which the heat index exceeded 30°C remained fairly steady, the numberofdaysexceeding34°Chasbeenincreasingasaproportionofthetotalnumberofdaysover 30°C. On average, the number of days per year exceeding 34°C has increased by approximately 5 days per decade. This implies that the number of very hot days is increasing faster than over all gradual temperature increases. Maximum day temperatures are increasing at an average rate of ~0.1°CperdecadeforDaNang,whichisconsistentwithotherstudiesNguyenetal.2014,Daoetal. 2013. Figure6:TrendsinhistoricalheatindexexceedingcriticalworkthresholdsspecifiedbyMOH.The numberofdaysperyearatthe30°Chasremainedfairlyconstant,butthenumberofdaysexceeding 34°Chasincreasedanaverageof5daysperdecade.Thenumberofveryhotdaysisincreasingfaster thanoveralltemperatureincreases. HotNights Thenumberofnightsinwhichtheheatindexexceeds28°Cremainedconstantover1970;1993,but appearstohavedecreasedaroundthemid;tolate;1990sseeFigure7.Overall,averagenighttime ambient temperatures have declined by about 0.3°C between 1970;2011, with most of the downwardtrendhappeningafter1995.

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19 70 ,7 4 19 75 ,7 9 19 80 ,8 4 19 85 ,8 9 19 90 ,9 4 19 95 ,9 9 20 00 ,0 4 20 05 ,0 9 Te m p e ra tu re C 5,YearAverageDayTemperatureandHeatIndex AmbientTemperature HeatIndexInside 150 170 190 210 230 250 270 290 310 330 350 197079 198089 199099 200009 D a y s Daysperyearofheatindexthreshold 30C 34C 14 InstituteforSocialandEnvironmentalTransition www.i7s7e7t.org We speculate that the downward trend seen since 1995 is partially due to the amount of data missing prior to this year, though some of it appears to be happening naturally as shown in the stationrecords.Therewasconsiderablemissingdailydatafromthestationdatasets 3 between1976; 1993 that we had to fill using the ERA;Interim datasets and some NCEP Reanalysis data for the period 1976;1978. We found that while the ERA datasets were highly correlated with the daily minimumtemperatures,theydidexhibitahotbiasthatcouldhaveartificiallyinflatedthenumberof veryhotnights.TheNCEPReanalysisdataisofverycoarseresolution~278.3kmspacingandwas usedtoapproximatemissingdays,alsointroducingabias.After1993,whenstationrecordsbecame more complete, the true number of very hot nights can be assessed and it does appear that the downwardtrenddoescontinue.Itisbeyondthescopeofthisstudytospeculatewhythisdownward trendexistsorwhatiscausingit.Wedonotexpectittocontinueinthefuturehowever,asclimate changeislikelytocausenightstowarmfasterthandays. Figure7:Trendsinaveragenighttimetemperaturesandaverageheatvaluesbetween1970;2011 leftfigureandtrendsinthenumberofveryhotnightswheretheheatindexexceeds28°Coverthe sameperiodrightfigure.Thedecliningtrendinveryhotnightsissignificantatthe95thpercentile andbeginsroughlyaround1995.Seeaccompanyingtextforpossibleexplanationofthedeclining trendinveryhotnights. 3 Duetolowstationdensity,datagapsasalegacyofthewarandrebuildingperiod,andthediversityof DaNang’sterraingoingfromseatomountainsoverashortdistance,itwasdifficulttorunsomeofthe standarddataqualitychecksonthehistoricaldataset.Homogenitytestsareneededtoremoveartificial trendsthatcancreepintostationdata,suchasitappearingtobemuchhotterorcooleroveraparticular periodifatreeorbuildingisputupneartheweatherstation.However,conductingsuchtestsand removingartificialtrendsrequiresbeingabletocompareenoughnear?byweatherstationsagainsteach otherandseeiftheyallshowthesametrendornot.Wecouldnotrunthesetestsfordatapriorto1993 duetolowstationdensityweonlyhadonestationforthewholeperiod,andpartialdatafrom2other stationsbetween1970?1993. BothERA?InterimandNCEPReanalysisaregriddeddatasetsinwhichobservationdatafrommultiple sources–weatherstations,ships,radiosonde,etc.–iscompiledandanalyzedtoproducearea?averaged climatedataforeachgridspace.Thespatialresolutionofeachgridisanapproximationofthewhole area’sclimate;localfeatureslikeDaNang’surbanheatislandareblurredinthedatasets.However,they aresometimestheonlywaytoapproximateanarea’shistoricalclimatewhenstationdataareincomplete orhardtoaccess.

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