Life expectancy rose substantially across the last century, and the years added have not been uniformly healthy ones. The gap between total years and healthy years is itself a measurable quantity.
Two different numbers
Lifespan counts years lived. Healthspan attempts to count years lived without significant disease or functional limitation, and it is estimated from survey and clinical data.
Healthspan is harder to measure because it requires a threshold for what counts as impairment, and different thresholds produce different figures for the same population.
Whatever the threshold, the two curves have not moved in step, and the interval between them has widened in many countries.
The early gains came from different causes
Most of the initial rise in life expectancy came from reducing deaths in infancy and childhood, through sanitation, nutrition, clean water and vaccination.
Preventing an early death adds a full lifetime of mostly healthy years, so those gains raised healthspan and lifespan together.
Once early mortality fell to low levels, further gains had to come from older ages, where the arithmetic works differently.
Treatment converted fatal events into chronic conditions
Later advances were largely in managing conditions that had previously killed quickly, including heart disease, stroke and several cancers.
These extended survival after diagnosis without necessarily delaying the diagnosis itself, so people live longer with a condition rather than longer before it.
That distinction is the mechanism behind the widening gap. Mortality from these conditions fell considerably faster than the rate at which people developed them in the first place.
Detection also moved earlier through screening and routine testing, which adds years lived with a diagnosis even where the course of the disease is unchanged.
Multiple conditions accumulate
Surviving one condition means remaining at risk of others, so people increasingly live with several simultaneously.
Each is managed separately by a different specialty, and the combination produces functional limitation greater than any single condition would.
This accumulation, rather than any single disease, is what defines the later portion of the gap for many people.
The compression question remains open
One long-standing proposal holds that illness could be compressed into a shorter period near the end of life if onset were delayed faster than death.
Achieving that requires interventions acting on the processes underlying multiple conditions at once, which is the stated aim of much current ageing research.
Whether the gap widens or narrows depends on which moves faster, and the evidence so far differs between countries and between social groups within them.