The milestone of the world’s population reaching 7 billion is a fitting time to reflect upon life expectancy. Life expectancy is a rough marker of humanity and medicine’s progress in the age-old battle to attain old age.

Those born in the US this year are expected to live until 78.37 years of age if current mortality rates persist. This puts us at a sobering 50th in the world.

How long is someone your age expected to live? It’s tempting, but erroneous, to calculate it as follows:

Current life expectancy – my current age = my expected remaining years

Why? For one, your overall life expectancy increases as you grow older; lumped into the life expectancy figure were those who died in infancy and childhood. A 30 year-old today is expected to live until 81.5 years; a 65 year-old until age 85. The key resource here is an actuarial life table, such as this one published by the Social Security Administration.  More detailed actuarial estimates will take into account your family history; your medical history; your smoking, eating, and drinking habits; your weight; and your education level; all of which contribute substantially.

Knowing how long a patient has to live can inform treatment options. The use of CT scans in children is kept to minimum, because the high levels of radiation can induce secondary tumors decades later. On the other hand, patients with terminal cancer sometimes undergo radiation therapy to relieve pain–their current cancer will prove fatal long before a cancer induced by the radiation. A patient’s estimated remaining number of years are factored into screening guidelines for breast and prostate cancer.

This increasing reliance on life expectancy brings up all kinds of fascinating ethical conundrums. If too many medical decisions become centered around life expectancy, life expectancy could become a self-fulfilling prophecy and an entrenched form of discrimination. Should a teenage male needing a kidney should get preference over an elderly woman, simply because he is expected to live longer? If so, should an African-American person get a kidney before a Native American? Should a richer person get a kidney before a poorer person? What about people with life-shortening diseases that hit certain ethnic groups more frequently (sickle-cell anemia, Huntington Disease, Gaucher disease, cystic fibrosis)?

Seeing how life expectancy changes over time gives us a sense of how far we have come, largely thanks to advances in nutrition, sanitation, and combating infectious disease. In 1850, US life expectancy at birth was 38 years, just under half what it is now. 38 years is also the current life expectancy of someone born in AIDS-ravaged Angola.

Why is the US life expectancy so low relative to other industrialized countries’, especially when our politicians claim we have the greatest medical system in the world? Monaco’s is 90! Main contributors are our obesity rate and smoking rate, or put another way, our government’s lack of emphasis on public health. Low-hanging fruit would include increasing cigarette taxes (which significantly decrease smoking rates) and tackling our burgeoning obesity problem.

78.37 years is a simple number with immense ramifications.

“Reflex Hammer” is a medical student who blogs at The Reflex Hammer.

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