Thermal　tolerance,　which　is　the　condition　of　mind　of　occupants　expressing　their　total　unacceptability　to　the　thermal　environment　after　physiological　and　psychological　regulation　and　any　behavioral　adjustments　other　than　air　conditioning,　plays　a　key　role　in　air　conditioner　operation　behavior　in　summer.　However,　literature　reviews　have　illustrated　that　systematic　analysis　of　thermal　tolerance　remains　a　key　challenge.　Previous　studies　have　mostly　focused　on　high　indoor　air　temperature,　but　did　not　reveal　the　nature　of　thermal　tolerance　based　on　the　relationship　between　the　indoor　environment,　air　conditioner　switch-on　behavior,　and　thermal　responses　of　the　human　body.　To　address　these　knowledge　gaps,　the　data　from　a　field　survey　of　54　occupants　in　Beijing　conducted　over　one　summer　were　used,　including　physical　measurements　of　environmental　parameters　and　occupants＇　thermal　sensations.　In　addition　to　showing　indoor　air　temperature　beyond　the　adaptive　thermal　comfort　zone,　this　study　provides　new　insights　on　thermal　tolerance　by　elucidating　the　temporal　deviation　between　air　conditioner　switch-on　and　occupants＇　thermal　unacceptability.　This　temporal　deviation　depends　strongly　on　the　ages　and　genders　of　occupants,　highlighting　the　nature　of　thermal　tolerance.　Moreover,　the　impacts　of　age　and　gender　were　demonstrated　to　be　more　reflected　in　differences　in　psychological　conditions　and　behavioral　and　cultural　backgrounds　than　in　physiological　conditions　alone.　Thermal　tolerance　was　thus　estimated　quantitatively　using　the　＂discomfort　capacity＂,　a　cumulative　value　of　the　difference　between　the　indoor　air　temperature　and　the　lower-limit　of　discomfort　temperature　during　the　period　of　thermal　discomfort　episodes　prior　to　occupants＇　turning　on　the　air　conditioning　in　summer,　to　account　for　the　combined　effects　of　both　the　environmental　stimulus　intensity　and　the　duration　of　discomfort　episodes　on　air-conditioner　switch-on.　On　this　basis,　for　young　male,　young　female,　middle-aged　male,　middle-aged　female,　and　elderly　occupants,　the　average　values　of　the　maximum　duration　of　discomfort　episodes　prior　to　occupants＇　turning　on　air　conditioning　at　various　indoor　air　temperatures　were　presented,　by　which　occupants＇　thermal　tolerance　to　hot　indoor　environment　in　summer　can　be　intuitively　quantified　from　both　the　temporal　dimension　and　the　intensity　of　stimulus.　This　study　will　be　helpful　to　optimize　the　operation　strategy　of　air　conditioner　by　minimizing　the　dependence　of　occupants　on　air　conditioning,　and　then　to　realize　energy　savings.