An Addendum on Solar Radiation, Reliability Theory, and Longevity
A few years ago, I pointed out some speculative work on environmental radiation during embryonic development and its possible effects on later longevity:
The amount of [solar] radiation varies according to where you are in the world, what time of year it is and cyclic changes in the sun’s behaviour. The Equator generally gets the most radiation, and in the northern hemisphere, the usual radiation peaks will be in June and July, but there will be variations from year to year according to "solar cycles." Every 11 years the Sun goes through a cycle when the magnetic field changes and the number of sunspots grows and dwindles. This affects the amounts of radiation produced. The Maine researchers suggest that high radiation levels either stress the immune system of embryos and foetuses or cause small mutations in their DNA, which can either predispose or protect from disease, mould brain characteristics and influence length of life.
The reliability theory of aging and longevity models complex organisms such as humans as an array of systems composed of many redundant component parts. It suggests that we are all born with a certain level of preexisting damage, and this early life damage load goes some way towards determining life expectancy. More damage at the outset means a greater likelihood of a shorter life.
I recently noticed a paper that takes the natural next step in this consideration of radiation, which is to look at the way in which cosmic radiation varies over time, and whether that might be correlated with damage to developing embryos. The level of cosmic radiation we experience at the Earth's surface is tied to the solar cycle, amongst other influences: the solar cycle is essentially an electromagnetic phenomenon, and the sun's electromagnetic fields shield against cosmic radiation to a level dependent on their current strength and configuration.
In this article, the author proposes to consider a link between infant mortality rate (IMR) and galactic cosmic radiation (CR) density. The periodical increase in solar activity increases the effect of the magnetic field of the sun, and therefore weakens galactic cosmic rays hitting the Earth's surface. As a result, embryos in their early stages of development may be less exposed to high-energy ionizing cosmic rays when the solar activity peaks. In the study discussed here, cosmic ray density data were correlated with the U.S. infant mortality rate in the following year. Statistical analysis shows that in the past 30 years, Pearson correlation between the change in galactic CR flux and IMR decrease in the following year was -0.36 (p < .05)
So not terribly correlated then - much like the prior studies on solar radiation, this suggests there might be a cause and effect in there somewhere, but it's small compared to other influences, and the data is noisy. Still, this is a good approach to identifying potential developmental factors that impact later longevity: when considered from the reliability theory point of view, any environmental condition that raises infant mortality rates should also reduce life expectancy for the survivors.
Shamir L (2010). Does cosmic weather affect infant mortality rate? Journal of environmental health, 73 (1), 20-3 PMID: 20687328