Cancer, heart disease and stroke are the main causes of death in Japan. In elderly population (65-84 years old) pneumonia follows next and the death from accidents come in fifth place.
Traffic accident comes in mind when it's called accident, though in fact more than half of incidents are non-traffic, and out of non-traffic accidents, increasing number of in-house drowning accidents draws attention. Drowning death incidents are categorized as accidents. However incidents caused by cardiac and cerebral attacks occurred during bathing are categorized as the deaths from illnesses. With those incidents counted, the number of deaths during bathing has been suggested to be enormous. When compared even with Western countries, number of in-home drowning accidents in Japan comes in the top position, and it's contrarily with the low ratio of other causes than drowning.
Although it's estimated that the number of sudden death during bathing is increasing, very little has been known. One of the reasons for this limited exposure is because most of the incidents occur when bathing alone with no witnesses to explain the circumstance. Another reason is that cause of death can't be diagnosed as drowning without definite or clear evidence, and diagnosis other than drowning won't surface as incidents related to bathing. Last reason is that autopsy doesn't always clarify cause of death.
Whatever the reasons, 80% of bathing accidents occur among healthy old people who are able to bath themselves, and this notion leads to speculation that they wouldn't have to be mortal accidents if not bathing. Therefore, bathing accidents during aging period are serious health issue.

Behind this large number of bathing accidents, Japan's unique social and cultural background come surface. For example, bathing for Western countries is mainly for hygiene purpose whereas in Japan for majority of people bathing serves purpose in deep warming up of body, relax and enjoyable time. It's reported that 100% of people in Japan soak in tub more than three times a week whereas it's only 23% in the U.S.(New York) (Shiseido Co., 1994). Moreover, according to an investigation, average Japanese businessmen defy bathing time as the most relaxing time at home.
On the other hand, in the U.S., many houses have multiple but shower-only bathrooms, and common use may be no tub soaking, just shower in the morning, before going out or before having guests. It's not at all unusual that there are people almost never soak in tub (Tokyo Gas Co.). There is an anecdote from the Crusade era tells how Moslems were shocked and said 'Westerners don't know how to bath nor have idea what hygiene is'. Whereas in Japan, public bath called Sentoh has been familiar place for socialization and information exchange, it shows very different spectrum about bathing from Western culture. So that in terms of mechanism of bathing accidents, it's much further complicated than effect of bathing and influence to physiological response.

Since physical environment such as temperature change is suspected to be the biggest trigger of bathing accident, let's explore three major effects of bathing; heating effect, water pressure effect and buoyancy effect.
Heating effect induces vasodilatation promoting blood flow to increase oxygen supply to tissues, and it decreases cardiac load as cardiac output increases. However with elderly, heating effect can work the opposite way depending on cardiac function level, and also the effect differs according to dressing room temperature or to water temperature. Blood vessels shrink in cold dressing room and blood tends to gather in heart, brain and organs like digestive ducts. In case of high temperature bathing(about 42°Cand above), blood flow to periphery significantly increases by heating effect right after getting into the tub. With atherosclerotic condition in heart or brain, significant decrease of blood flow can lead ischemia in those organs. Also, longer soaking in high temperature water decreases amount of blood by perspiration and accelerates coagulation of blood that may trigger cerebral or myocardial infarction.
Blood pressure changes significantly during bathing. Blood pressure is already on the rise in a cold dressing room and as soon as soaking in high temperature water it increases even more. Then blood pressure starts to decrease and about 4~5 minutes after soaking systolic blood pressure goes down to 5~30% lower than pre-bathing blood pressure. Blood pressure change is more radical with elderly, hypertension, longer soaking, or when water temperature is higher. In some cases even more blood pressure decrease may happen with the motion standing up as getting out of the tub.
As often seen in elderly, this is suspected to happen with the same mechanism as postprandial hypotension or orthostatic hypotension. The important inducing factors responsible to consciousness disturbance and bathing hot flash that result drowning and drowning down.
In still water, water pressure, which is in proportion to the air pressure and the depth, so called still water pressure (hydrostatic pressure), is in charge. Legs are to bring blood back to the heart and this important role is called 'Leg Pumping'.
Because legs are positioned deeper in the water, venous blood would be flown back to the heart by added still water pressure, and in result cardiac output increases.
However when soaking whole body, by the added still water pressure, the girth of the chest shrinks 1~3cm and the girth of the abdomen shrinks 3~5cm. With the pressured thorax and shrunken girth of the chest, diaphragm is pushed up. And that may result in less blood flow from the heart to the lung and more load to the heart. The heart work is assumed less when soaking only lower body.
The effect of buoyancy to physiological reaction has not been clear. With lightened weight, it's applicable to rehabilitation in the heated pool.

Since the bathroom is a private space, there hardly has been any major research on accidents during bathing. This time with the support of Tokyo Metropolitan Fire Department, based on the result from investigation done from October 1999 to March 2000, cases on sudden death during bathing in Japan were examined. Within 23 districts in Tokyo, there were 628 accidental deaths in 6 months, estimated 866 deaths in a year. With the consideration of age-composition difference in Tokyo and national population, it's calculated to be 14,000 sudden deaths during bathing in Japan in 1999. The number is larger than that of traffic accidents, and in elderly population, is even multiply larger than that of traffic accidents.
Also they were already dead (cardio-pulmonary arrest) at the site in half of the cases. Factors related to sudden death during bathing were analyzed, and as shown in Table 1, more advanced age, female, colder temperature and reported in the early morning are the characteristics that have tendency with mortality. On the other hand, there were fewer deaths in the cases that were occurred in the early evening. Specific information why more deaths during bathing had seen in female haven't been clear, however the relationship between the time of the accident and mortality has an interesting point in terms of human biological rhythm. Vital rhythms such as body temperature and blood pressure have its rhythm in 24-hour period, and it's known that it has the highest value in the late afternoon or in the early evening and the lowest in the early morning.

Considering what has discussed above, the mechanism of sudden death during bathing can be explained as follows. By the effects from temperature such as atmosphere temperature, room temperature, and water temperature, and from water pressure, cardiac and vascular reactivity or an attack occurs followed by consciousness disturbance. If it's in the tub, it leads to drowning or drowning death accident, and if it's outside of the tub, it leads to fall accident or bathing hot flash.

Figure. Mechanisms on sudden death during bathing