In 1986, the Soviet minister of hydrometeorology, Yuri Izrael, had a regrettable decision to make. It was his job to track radioactivity blowing from the smoking Chernobyl reactor in the hours after the 26 April explosion and deal with it. Forty-eight hours after the accident, an assistant handed him a roughly drawn map. On it, an arrow shot north-east from the nuclear power plant, and broadened to become a river of air 10 miles wide that was surging across Belarus toward Russia. If the slow-moving mass of radioactive clouds reached Moscow, where a spring storm front was piling up, millions could be harmed. Izrael’s decision was easy. Make it rain.

So that day, in a Moscow airport, technicians loaded artillery shells with silver iodide. Soviet air force pilots climbed into the cockpits of TU-16 bombers and made the easy one-hour flight to Chernobyl, where the reactor burned. The pilots circled, following the weather. They flew 30, 70, 100, 200km – chasing the inky black billows of radioactive waste. When they caught up with a cloud, they shot jets of silver iodide into it to emancipate the rain.

Midnight in Chernobyl and Manual for Survival review – the hidden story uncovered Read more

In the sleepy towns of southern Belarus, villagers looked up to see planes with strange yellow and grey contrails snaking across the sky. Next day, 27 April, powerful winds kicked up, cumulus clouds billowed on the horizon, and rain poured down in a deluge. The raindrops scavenged radioactive dust floating 200 metres in the air and sent it to the ground. The pilots trailed the slow-moving gaseous bulk of nuclear waste north-east beyond Gomel, into Mogilev province. Wherever pilots shot silver iodide, rain fell, along with a toxic brew of a dozen radioactive elements.

If Operation Cyclone had not been top secret, the headline would have been spectacular: “Scientists using advanced technology save Russian cities from technological disaster!” Yet, as the old saying goes, what goes up must come down. No one told the Belarusians that the southern half of the republic had been sacrificed to protect Russian cities. In the path of the artificially induced rain lived several hundred thousand Belarusians ignorant of the contaminants around them.

The public is often led to believe that the Chernobyl exclusion zone, a depopulated 20-mile circle around the blown plant, safely contains Chernobyl radioactivity. Tourists and journalists exploring the zone rarely realise there is a second Chernobyl zone in southern Belarus. In it, people lived for 15 years in levels of contamination as high as areas within the official zone until the area was finally abandoned, in 1999.

In believing that the Chernobyl zone safely contained the accident, we fall for the proximity trap, which holds that the closer a person is to a nuclear explosion, the more radioactivity they are exposed to. But radioactive gases follow weather patterns, moving around the globe to leave shadows of contamination in shapes that resemble tongues, kidneys, or the sharp tips of arrows.

England, for example, enjoyed clear weather for several days after the Chernobyl accident, but rain started on 2 May, 1986 and fell heavily on the Cumbrian fells – 20mm in 24 hours. On the uneven, upland terrain, radioactive fallout pooled in rivulets and puddles. The needles on radiation detectors at the Sellafield (formerly Windscale) nuclear processing plant went upwards alarmingly, 200 times higher than natural background radiation. From 5 becquerels a square metre, radiation levels in topsoil spiked to 4,000 bq/m2. Kenneth Baker, the then environment secretary, issued assurances that the radioactive isotopes would soon be washed away by rain.

Facebook Twitter Pinterest Ukrainians protest against the cover-up of the consequences of the Chernobyl accident, April 1990. Photograph: Игорь Костин/РИА Новости

Two months later, however, levels rose yet higher to 10,000 bq/m2 in Cumbria and 20,000 bq/m2 in south-western Scotland, 4,000 times higher than normal. Scientists tested sheep and found their levels of caesium-137 were 1,000 becquerels per kilogram – too high for consumption. In the midst of general anxiety, the Ministry of Agriculture, Fish and Food (MAFF) issued temporary restrictions on the sale of meat for 7,000 farms.

The early predictions of caesium being washed from upland soils proved optimistic. The mineral-starved native plants efficiently drank up radioactive isotopes. Tiny micro-fungi moved caesium-137 from the roots to plant tips, where grazing sheep fed.

Researchers added months, then years, to their predictions of how long the radioactive caesium would linger in the environment. Eventually, restrictions remained in place for 334 farmers of north Wales for 26 years.

As researchers monitored Chernobyl radioactivity, they made a troubling discovery. Only half of the caesium-137 they detected came from Chernobyl. The rest had already been in the Cumbrian soils; deposited there during the years of nuclear testing and after the 1957 fire at the Windscale plutonium plant. The same winds and rains that brought down Chernobyl fallout had been at work quietly distributing radioactive contaminants across northern England and Scotland for decades. Fallout from bomb tests carried out during the cold war scattered a volume of radioactive gases that dwarfed Chernobyl.

Revisiting Chernobyl: 'It is a huge cemetery of dreams' Read more

The Chernobyl explosions issued 45m curies of radioactive iodine into the atmosphere. Emissions from Soviet and US bomb tests amounted to 20bn curies of radioactive iodine, 500 times more. Radioactive iodine, a short lived, powerful isotope can cause thyroid disease, thyroid cancer, hormonal imbalances, problems with the GI tract and autoimmune disorders.

As engineers detonated 520 nuclear bombs into the atmosphere, scientists lost track of where radioactive isotopes fell and where they came from, but they caught glimpses of how readily radioactivity travelled the globe. In the 1950s, British officials detected harmful levels of radioactive caesium in imported Minnesota wheat. The wheat became radioactive from US bomb tests in Nevada, 2,500km from the Minnesota wheatfields. But over the years, scientists failed to come to an agreement on what the global distribution of radioactivity in the food chain did to human health. When the Chernobyl accident occurred, experts in radiation medicine called for a long-term epidemiological study on Chernobyl-exposed people. That study never occurred. After Fukushima, Japanese scientists said what Soviet scientists asserted after Chernobyl – we need 20 years to see what the health effects from the accident will be.

Fortunately, Chernobyl health records are now available to the public. They show that people living in the radioactive traces fell ill from cancers, respiratory illness, anaemia, auto-immune disorders, birth defects, and fertility problems two to three times more frequently in the years after the accident than before. In a highly contaminated Belarusian town of Veprin, just six of 70 children in 1990 were characterised as “healthy”. The rest had one chronic disease or another. On average, the Veprin children had in their bodies 8,498 bq/kg of radioactive caesium (20 bq/kg is considered safe).

For decades, researchers have puzzled over strange clusters of thyroid cancer, leukaemia and birth defects among people living in Cumbria, which, like southern Belarus, is an overlooked hotspot of radioactivity from cold war decades of nuclear bomb production and nuclear power accidents.

Britain’s nuclear U-turn puts us in a very lonely club | Fred Pearce Read more

Currently, policymakers are advocating a massive expansion of nuclear power as a way to combat climate change. Before we enter a new nuclear age, the declassified Chernobyl health records raise questions that have been left unanswered about the impact of chronic low doses of radioactivity on human health. What we do know is that as fallout from bomb tests drifted down mostly in the northern hemisphere, thyroid cancer rates grew exponentially. In Europe and North America, childhood leukaemia, which used to be a medical rarity, increased in incidence year by year after 1950. Australia, hit by the fallout from British and French tests, has one of the highest incidence rates of childhood cancer worldwide. An analysis of almost 43,000 men in North America, Europe, Australia, and New Zealand, showed that sperm counts dropped 52% between 1973 and 2011.⁠

These statistics show a correlation between radioactive contaminants and health problems that are similar to those that materialised in Chernobyl-contaminated territories. A correlation does not prove a causal link. These statistics do, however, invite a lot of questions; questions that scientists and stakeholders should tackle before we enter a second nuclear age.

• Kate Brown is a historian of environmental and nuclear history at Massachusetts Institute of Technology. Her new book is Manual for Survival: A Chernobyl Guide to the Future

• This article was amended on 4 April 2019 to correct the number of men whose sperm counts were analysed from “almost 43,00” to almost 43,000. It was further amended on 17 May 2019 to correct the number of nuclear bombs detonated at that time to 520, from 2,000 which was the total number of total bomb tests.