As a heat wave continues to grip much of Japan and send thousands to hospital with heat-related illnesses, medical workers worry that the similarity of symptoms to COVID-19 may place extra pressure on a health care system already creaking under the strain of the coronavirus pandemic.

The number of people showing signs of heatstroke or heat exhaustion has sharply increased recently. Temperatures soared to 41.1 C in Hamamatsu in central Japan on Monday, tying with the country's highest-ever temperature, marked in Kumagaya near Tokyo in 2018.

"There are times when we can't immediately tell apart (those suffering from heat-related illness and COVID-19) when a (patient) is feeling unwell with high fever because it is a symptom they have in common," said Yasufumi Miyake, head of the advanced emergency medical service center at Teikyo University Hospital.

Miyake said treatment has to take account of the possibilities of both COVID-19 and heat-related conditions when staff cannot rule out the possibility of coronavirus infection. "There is a risk that the medical system will collapse" if this situation continues, he added.

Amid fears that mask-wearing to prevent the spread of the novel coronavirus could itself cause heatstroke or heat exhaustion, 12,804 people were taken to hospital across Japan between Aug. 10 and Aug. 16 for heat-related conditions, up from 6,664 people the previous week, according to the Fire and Disaster Management Agency.

People walk with parasols in Kyoto, western Japan, on Aug. 19, 2020, amid scorching summer heat. (Kyodo)

In central Tokyo, the death toll from heatstroke or heat exhaustion in August stood at 79 as of Tuesday, compared with zero through July, with 73 of them, or about 90 percent, aged 60 or older, according to a tally by the Tokyo Medical Examiner's Office.

While scorching heat has been a problem in Japan in recent years amid global warming, experts have suggested that the longer-than-usual rainy season this year with particularly overcast weather made it harder for people to physically adapt to the sudden rise in temperatures this month.

During the rainy season, many regions in Japan lacked sunlight in July and people could not gradually get used to hotter weather, they said.

Hiroyuki Kusaka, a professor of meteorology at the University of Tsukuba, said the recent heat wave has been caused by two high-pressure systems that have overlapped over Japan.

To prevent the risk of heat-related illness, Miyake emphasized the need to keep cool and hydrated. He especially called on people to take "extra care" of the elderly, who are considered the most vulnerable to the heat.

People should not only urge seniors to turn on air conditioning but also check up on whether they have done so, he said.