In August 1945, the Second World War was drawing to a close after atomic bombs were dropped on the Japanese cities of Hiroshima and Nagasaki, forcing a surrender. But at Los Alamos National Laboratory, where the bombs had been developed, scientists were still conducting perilous experiments on a plutonium core reserved for another possible nuclear use should the war continue.
Experiments were conducted by hand, before later accidents forced far stricter safety standards, and on 21 August 1945, physicist Harry Daghlian accidentally dropped a piece of tungsten carbide onto the plutonium core. Daghlian tried to halt the reaction with his bare hands and received a high dose of radiation. Less than a month later, he was dead. In May 1946, physicist Louis Slotin had an accident during an experiment on the same core, when his screwdriver slipped, triggering a powerful flash of radiation. He died nine days later.
The tragedies offered an early lesson in what ionising radiation can do to the human body in a matter of seconds. Nearly four decades later, in the early hours of 26 April 1986, the Chernobyl nuclear power station disaster in Ukraine revealed another face of radiation exposure, this time outside a military laboratory. After Reactor No 4 at Chernobyl exploded, firefighters rushed to the scene without fully grasping the nature of the invisible danger before them.
Among them was Vasily Ignatenko, a 25-year-old Soviet firefighter who helped fight the initial fires. He received a high dose of radiation and developed acute radiation syndrome. He died two weeks later, on 13 May. His wife, Lyudmila, later recounted his final days in testimonies documented by the Belarusian writer Svetlana Alexievich, winner of the 2015 Nobel Prize in Literature, in her book Chernobyl Prayer. What Daghlian, Slotin, and Ignatenko had in common was exposure to large doses of ionising radiation. What is this composed of, and how does it affect the human body?
Essence of danger
Radiation is generally divided into two broad categories. The first is non-ionising radiation such as radio waves, visible light, and infrared rays, which usually lacks sufficient energy to strip electrons from atoms. The other is ionising radiation, such as gamma rays, alpha and beta particles, neutrons, and certain X-rays, which can separate electrons from atoms and turn them into ions. This capacity is the essence of the danger, because it allows radiation to alter the chemical structure of living molecules, above all DNA, the molecule that carries genetic instructions inside human cells.
James Watson and Francis Crick revealed DNA’s double-helix structure in 1953. The molecule consists of two strands coiled around each other, each made up of small units known as nucleotides. These units carry four nitrogenous bases: adenine, thymine, guanine and cytosine (known by most as A, T, G, and C). Adenine pairs with thymine, and guanine with cytosine, in a precise arrangement resembling the rungs of a ladder. From the order of these bases arise the genes that instruct the cell to produce proteins and regulate the functions of the body.
Although this system is highly precise, it is not as fragile as it may seem. DNA sustains limited damage every day from natural processes within the body, including oxidation and errors that occur when genetic material is copied during cell division. For this reason, cells have lots of repair mechanisms, including to repair or correct damaged bases, mismatches during copying, and breaks in one DNA strand or in both. These correcting mechanisms have limits, however, and when damage is extensive, complex, or concentrated within a short period of time, accurate cell repair may be impossible.
Ionising radiation can damage DNA in two ways. The damage may be direct, as radiation strikes the DNA molecule itself and breaks its chemical bonds, or it may be indirect. The latter is a pathway of particular importance in human cells, because most of the cell is water. When radiation ionises water molecules, highly reactive free radicals are formed. These can attack DNA, proteins, and lipids inside the cell. As a result, radiation need not strike DNA directly to cause serious genetic damage.
Repairing the damage
Damage to DNA can range from small changes in its chemical bases to breaks in a single strand and, more dangerously, breaks in both strands at once. Double-strand breaks are among the most serious forms of damage, because the cell is forced to reconnect two severed ends in a molecule whose information is arranged with extraordinary precision. If the ends rejoin incorrectly, mutations may occur, parts of the DNA may be lost, genes may fuse abnormally, or chromosomes may be rearranged.
The cell then faces several possible outcomes: it may repair the damage and return to normal function; it may fail to repair and die (which can itself serve as a protective mechanism when the damage is severe); or it may survive while carrying a mutation or chromosomal defect. In such cases, the consequences do not always appear immediately. The danger may extend over years or even decades, especially if the mutations affect genes that control cell division, DNA repair, or tumour suppression.
