His post-war scientific work was in the field of nuclear medicine, with an emphasis on the use of radioisotopes to study red blood cells in humans.

He developed an interest in how red blood cells respond to physiological stress, such as exposure to high elevations.

In 1957, he participated in a joint American-British Antarctic expedition which studied the effects of extreme cold on human blood.

More than two-thirds of the earth's atmosphere lies below its summit, and for an unacclimatized man without oxygen, the top of the mountain is more endurable than outer space by only two or three minutes.

[8] Siri died of pneumonia at his home in Berkeley, California after suffering from Alzheimer's disease for a decade.