membrane using infrared sensors and also aim to measure core body temperature, since the blood supply of this membrane is directly shared with the brain. however, this method of measuring body temperature is not as accurate as rectal measurement and has a low sensitivity for fever, failing to determine three or four out of every ten fever measurements in children. ear temperature measurement may be acceptable for observing trends in body temperature but is less useful in consistently identifying and diagnosing fever. until recently, direct measurement of core body temperature required either an ingestible device or surgical insertion of a probe. therefore, a variety of indirect methods have commonly been used as the preferred alternative to these more accurate albeit more invasive methods. the rectal or vaginal temperature is generally considered to give the most accurate assessment of core body temperature, particularly in hypothermia. in the early 2000s, ingestible thermistors in capsule form were produced, allowing the temperature inside the digestive tract to be transmitted to an external receiver; one study found that these were comparable in accuracy to rectal temperature measurement. more recently, a new method using heat flux sensors have been developed. several research papers show that its accuracy is similar to the invasive methods. temperature variation hot 44 °c (111.2 °f) or more – almost certainly death will occur; however, people have been known to survive up to 46.5 °c (115.7 °f). 43 °c (109.4 °f) – normally death, or there may be serious brain damage, continuous convulsions, and shock. cardio-respiratory collapse will likely occur. 42 °c (107.6 °f) – subject may turn pale or remain flushed and red. they may become comatose, be in severe delirium, vomiting, and convulsions can occur. 41 °c (105.8 °f) – (medical emergency) – fainting, vomiting, severe headache, dizziness, confusion, hallucinations, delirium, and drowsiness can occur. there may also be palpitations and breathlessness. 40 °c (104 °f) – fainting, dehydration, weakness, vomiting, headache, breathlessness, and dizziness may occur as well as profuse sweating. 39 °c (102.2 °f) – severe sweating, flushed, and red. fast heart rate and breathlessness. there may be exhaustion accompanying this. children and people with epilepsy may suffer convulsions at this temperature. 38 °c (100.4 °f) – (classed as hyperthermia if not caused by a fever) – feeling hot, sweating, feeling thirsty, feeling very uncomfortable, slightly hungry. if this is caused by fever, there may also be chills. normal 36.5–37.5 °c (97.7–99.5 °f) is a typically reported range for normal body temperature. cold 36 °c (96.8 °f) – feeling cold, mild to moderate shivering. body temperature may drop this low during sleep. this can be a normal body temperature for sleeping. 35 °c (95 °f) – (hypothermia is less than 35 °c (95 °f)) – intense shivering, numbness and bluish/grayness of the skin. there is the possibility of heart irritability. 34 °c (93.2 °f) – severe shivering, loss of movement of fingers, blueness, and confusion. some behavioral changes may take place. 33 °c (91.4 °f) – moderate to severe confusion, sleepiness, depressed reflexes, progressive loss of shivering, slow heartbeat, shallow breathing. shivering may stop. the subject may be unresponsive to certain stimuli. 32 °c (89.6 °f) – (medical emergency) – hallucinations, delirium, complete confusion, extreme sleepiness that is progressively becoming comatose. shivering is absent (subject may even think they are hot). reflex may be absent or very slight. 31 °c (87.8 °f) – comatose, very rarely conscious. no or slight reflexes. very shallow breathing and slow heart rate. possibility of serious heart rhythm problems. 28 °c (82.4 °f) – severe heart rhythm disturbances are likely and breathing may stop at any time. the person may appear to be dead. 24–26 °c (75.2–78.8 °f) or less – death usually occurs due to irregular heart beat or respiratory arrest; however, some patients have been known to survive with body temperatures as low as 13.7 °c (56.7 °f). there are non-verbal corporal cues that can hint at an individual experiencing a low body temperature, which can be used for those with dysphasia or infants. examples of non-verbal cues of coldness include stillness and being lethargic concerning kinesiological movement, sneezing, unusual paleness of skin among light-skinned people, and, among males, shrinkage, and contraction of the scrotum. historical understanding in the 19th century, most books quoted "blood heat" as 98 °f, until a study published the mean (but not the variance) of a large sample as 36.88 °c (98.38 °f). subsequently, that mean was widely quoted as "37 °c or 98.4 °f" until editors realized 37 °c is equal to 98.6 °f, not 98.4 °f. the 37 °c value was set by german physician carl reinhold august wunderlich in his 1868 book, which put temperature charts into widespread clinical use. dictionaries and other sources that quoted these averages did add the word "about" to show that there is some variance, but generally did not state how wide the variance is.