by Paris Bentley
figures by Michael MacArthur
The immediate environmental effects of nuclear bomb testing during the Cold War era were undoubtedly devastating. Having left enormous negative environmental and socioeconomic impacts all over the world, it is hard to imagine that any sort of silver lining to these tests could exist. But despite all the destruction that these tests caused, their remnants are now being used to answer questions in biology that might otherwise have been unsolvable or, at the least, extremely difficult to study. Indeed, nuclear bombs set off in the 1950s and 1960s left a distinct environmental signature that is now being used to determine why certain body parts heal better than others, how often various tissues are replaced as you age, and providing us greater insight into the basis of many aging-related diseases.
Atomic bomb testing resulted in an influx of carbon-14 into the environment
Atomic bomb testing resulted in a huge influx of carbon-14 into the atmosphere. Carbon is a key component of many of the most intricate structures in our universe, from diamonds to DNA. Carbon-14 is an extremely rare form of carbon, referred to as a radioactive isotope that has 8 neutrons instead of the usual 6 (Figure 1).
While carbon-14 exists naturally at extremely low levels, several excess tons were generated from the above ground testing of hundreds of nuclear bombs in the 1950s and 1960s. Unfortunately, while these tests were performed in remote areas, their effects were not confined to their respective detonation sites. Within a few years, the excess carbon-14 was distributed evenly throughout the Earth’s atmosphere, peaking in 1963. This mid-20th century fluctuation is termed the “carbon-14 bomb pulse” (Figure 2). The influx of carbon-14 into the atmosphere also led to increased carbon-14 levels in all living things, including plants, animals, insects, and humans. Since nuclear bomb testing was banned in the early 1960s, carbon-14 levels have steadily been on the decline.
How is carbon-14 used to date human tissue?
Carbon-14 is absorbed by humans through a variety of sources including inhalation, drinking water, and diet. Radioactive isotopes, like carbon-14, are unstable and decay over time. It has a half-life of 5,700 years, meaning that the total amount of carbon-14 is halved over the course of 5,700 years. Every time a cell divides, it first replicates its genetic material, DNA, and this DNA incorporates a significant amount of carbon, which can include carbon-14 (Figure 3). Since carbon-14 takes so long to decay, it can be assumed that the amount of it in a molecule of DNA will remain the same as long as the cell does not divide. Thus the amount of carbon-14 present in human tissue is reflected by the levels in the environment at the time that that tissue was formed. Tissues with a high turnover rate will have carbon-14 levels similar to current atmospheric levels, and tissues with a low turnover rate will reflect previous atmospheric levels. Analysis of the carbon-14 to carbon ratio can reveal when a cell division occurred with a very high degree of accuracy, because unlike other components of cells, DNA remains static. This enables scientists to use carbon-14 concentrations to estimate the age of tissues for people who lived during and after the carbon-14 bomb pulse. Thus the influx of this carbon isotype that resulted from the detonation of hundreds of atomic bombs is allowing scientists to determine whether a human tissue remains static throughout one’s lifetime, as does the eye lens, or is renewed relatively often, like fat tissue.
What has carbon-14 dating taught us about the human body?
Why tendons and joints aren’t healing
Joints and tendons are frequently injured and associated with chronic pain and poor healing. Carbon-14 dating studies have revelead that the Achilles tendon is not renewed after the first 17 years of life and has poor regenerative capacity. Similarly, a recent study on the articular cartilage of the human knee found that there is essentially no turnover of this joint cartilage once an individual reaches adulthood. In both healthy and osteoarthritic individuals, carbon-14 levels in cartilage collagen consistently corresponded to the levels present in the atmosphere when that individual was between the ages of 8 and 13. This suggests that cartilage collagen of the knee is formed during these years, with little capacity for turnover or regeneration after an individual matures. Even disease or injury did not trigger regeneration of this tissue. These studies suggest that these tendons and joints are formed during an individual’s height growth and may even bolster the argument for avoiding physical activities in which injuries to these body parts are common, as they may inevitably lead to tendon or knee replacement surgery later in life.
How to treat aging-related diseases
Bomb-pulse dating methods have also revealed the basis for many aging-related diseases, including heart disease, Alzheimer’s disease, osteoarthritis, and lung disease. Prior to the use of carbon-14 dating, the cells of human heart muscle (called cardiomyocytes) were assumed to be form during the first weeks of life and were thought to be unable to regenerate, with loss of these cells being a major cause of heart failure. Bomb-pulse dating has revealed that these cells are actually replaced very slowly throughout adulthood, although the rate at which they are replaced does decrease with age. Renewal of these cells, which was previously thought to be impossible, would be huge step towards the cure of heart disease.
Is carbon-14 bad for you?
Carbon is estimated to make up approximately 18% of a human’s body weight. In a typical person, one out of every 1018 carbon atoms is carbon-14. There was an observed increase in the rate of childhood cancers during the years of the carbon-14 bomb pulse but it is uncertain how much carbon-14, in particular, contributed to this rise. Nuclear bombs release many other types of radioactive atoms which are also believed to play roles in causing cancer, however the exact amount of these atoms it takes to lead to a particular cancer in humans is unknown. While there is no safe dose of radiation, radioactive events take place in our bodies every second without causing cancer. It probably helps that cells have several mechanisms guarding against the growth of damaged cells.
A fading resource
Despite the devastation of cold-war era nuclear weapons testing on the environment, it has led to the answers to many important biological questions regarding cell regeneration and tissue renewal. No other technique has been identified that has the power to analyze cell turnover over the span of several decades with such high precision. Thus, the knowledge obtained from the unfortunate events of the cold-war era can lead to advances in the treatment of aging-related illnesses, while also revealing why it would be better to break your leg rather than your knee during your next snowboarding trip.
While we still have a lot to learn, we may not have much time to learn it. The human body is made up of dozens of different organs and types of tissues, but we probably only have a couple more decades before carbon-14 levels return to baseline. Thus, this method of estimating tissue age and turnover will not be possible in the future, unless we decide to test more atomic bombs above ground, but that’s probably not a good idea.
Paris Bentley is a PhD student in the Virology Program at Harvard Medical School.