Does proximity to nuclear power plants raise leukaemia risk? COMARE says no.Venture Media Group/Aurora Photos/Corbis

Living close to a nuclear power station does not increase a child's risk of developing leukaemia, according to one of the most detailed studies of the issue to date.

The latest report from the UK's Committee on Medical Aspects of Radiation in the Environment (COMARE), published today, calls into question the findings of an influential study in Germany which claimed that clusters of leukaemia cases were more likely to be found near nuclear plants.

Alex Elliott, chair of COMARE and a clinical physicist at the University of Glasgow, UK, says that researchers should now look for other causes of these leukaemia clusters.

Nuclear sites have long been plagued by allegations that they release sufficient low-level radiation to raise the risk of childhood cancers. Despite sparse evidence to support the hypothesis, public concerns have persisted.

The latest report from COMARE looks specifically at the incidence of this cancer in children under the age of 5 living near 13 of the UK's nuclear power plants. It found "no significant association", says Elliott.

Roughly 500 children in the United Kingdom are diagnosed with leukaemia in any given year, and over the entire study period of 35 years there were only 20 cases in children under 5 years of age within 5 kilometres of a power plant. "It's something that's very difficult to study statistically," admits Elliott.

As the numbers involved in this are tiny, it is possible that there could be an effect too small to measure, but this effect would itself be tiny in consequence, the authors say.

The COMARE report contrasts with a German study called Kinderkrebs in der Umgebung von Kernkraftwerken (KiKK; Childhood cancer in the vicinity of nuclear power plants). In 2008, the KiKK team reported¹that living within 5 kilometres of a nuclear power plant in Germany roughly doubled the risk of childhood leukaemia.

COMARE's latest report includes a critique of the KiKK study, pointing out that it did not fully assess confounding factors other than radiation, which may have been partly responsible for the association. In leukaemia, Elliott notes, there are known to be correlations between the disease and socioeconomic status and population density, for example. COMARE attempted to compensate for these and other issues, but the KiKK study did not. "If there was an effect like the KiKK study [in the UK] we would have found it and we didn't," says Elliott. Nature was unable to obtain comment from the KiKK team.

Cluster causes

The COMARE report also looked at leukaemia cases at several sites that had been proposed as locations for nuclear power plants that were never actually built. There was a small increase in childhood leukaemia cases within 5 kilometres of one of the proposed power plants, which may highlight the dangers of apparently statistically robust results occurring by chance when dealing with such rare diseases. Or, the report notes, it could mean that sites suitable for nuclear power plants have some characteristic which can also increase the risk of leukaemia.

Not everyone is convinced that the COMARE report settles the issue. Steve Wing, an epidemiologist at the University of North Carolina at Chapel Hill, says that studies to date have not been properly designed to detect any effect from the very low radiation doses that people living near nuclear plants may be exposed to. But a US team is now preparing to update a 1990 study on the link between nuclear power stations and cancer cases at the request of the US Nuclear Regulatory Commission (see 'US radiation study sparks debate'). Wing suggests that the US study could provide a more effective analysis by assessing the radiation doses that are received in utero and in early childhood by those living close to nuclear plants².

Others say there are alternative explanations for leukaemia clusters, such as the possibility of an infectious agent being responsible for triggering leukaemia.

Leo Kinlen, an epidemiologist based at the University of Oxford, UK, has long argued that sites such as nuclear power stations and military bases may be more likely to host leukaemia clusters because of the relatively high population turnover in those areas. Population mixing would increase people's exposure to novel viruses to which they had no immunity, potentially raising the risk of related illnesses, he says.

Mel Greaves, team leader in the biology of childhood leukaemia at the Institute of Cancer Research in Sutton, UK, instead suggests that the cases may arise from an abnormal immune response to common infections, rather than a specific virus. "I think the consensus view is that it's something to do with timing of exposure to infection," he says. 

• References

  1. Kaatsch, P. et al. Int. J. Cancer 1220, 721-726 (2008).
  2. Wing, S., Richardson, D. & Hoffmann, W. Environ. Health Perspect.119, 1-5 (2011). | PubMed |

These fossils suggest that some ancient marsupials lived in groups, unlike their modern relatives.Picture © Lemzaouda/MNHN; reconstitution © Fernandez/MNHN

Modern mammals often live in groups, but most marsupials are solitary. With no fossil evidence to suggest that the animals have ever behaved otherwise, palaeontologists have long assumed that marsupials have been loners throughout their evolutionary history. This notion is now being overturned by the analysis of a fossil site containing many marsupials that seem to have been living together.

The site, in the Tiupampa locality of Bolivia, contains 35 specimens ofPucadelphys andinus, a primitive opossum from the early Palaeocene Epoch (64 million years ago).

Teeth are usually all that palaeontologists can find of ancient mammals, because dentition is built to endure punishment, and fossilizes well. However, 22 of the 35 specimens at the Bolivian site consist of teeth, skulls and body skeletons in near-perfect shape.

Sandrine Ladevèze, a palaeontologist at the Royal Belgian Institute of Natural Sciences in Brussels, and her colleagues publish an analysis of the specimens today in Nature¹.

"To find a sample of this quality is almost unheard of," says Richard Cifelli, a palaeontologist at the University of Oklahoma in Norman.

Full house

But it is not the condition, but the placement of the specimens at the fossil site that intrigued Ladevèze.

She and her team report that the marsupial specimens are fossilized in two clusters, neither covering an area larger than one square metre, with 12 nestled together at one location and 23 at another, just 3 metres away.

Natural forces such as the flow of river water could have brought these specimens together after they died, but Ladevèze considers this unlikely, because skeletons that are moved after death are rarely found in such good condition. Instead, Ladevèze proposes that the marsupials were living together in two burrows when some sort of disaster, such as a flash flood, buried them alive.

"Sociality of this sort has been seen in fossils of other groups before [such as dinosaurs], but this is the earliest example of social gathering in marsupials that we have ever seen," says Zhe-Xi Luo, a palaeontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

The composition of the social groups inside the two burrows is very different from that seen in opossums today. Only a very few modern opossums will live in one territorial area, yet the morphology of the Bolivian specimens suggests that there were at least 6 adult males — identified by their large canines and long skulls — 12 adult females, 4 sub-adults and one immature individual present at the site.

"That such a mix of marsupials lived together hints that their modern solitary lifestyle was not present 64 million years ago," says Ladevèze.

"The team's ability to identify these fossils as males, females and juveniles of the same species is just fabulous. Had I found these fossils as isolated specimens, I shudder to think how many species I would have classified them as," says Cifelli.

Dead end

However, Marcelo Sánchez, a palaeontologist at the University of Zurich in Switzerland, questions whether the Bolivian find is indicative of general marsupial behaviour during the Palaeocene.

"It is possible that the gregariousness seen in these early marsupials was an evolutionary 'experiment' on a line which is not ancestral to living marsupials, and solitary behaviour was the ancient condition," he says.

Regardless of whether sociality was rampant among other species, it clearly applied to these Bolivian opossums. Yet what drove this urge to live in groups is a mystery.

Ladevèze initially speculated that a need for warmth might have drawn the animals together. Indeed, some typically solitary marsupials in South Australia snuggle together in groups of up to 40 to stay warm when temperatures fall in winter. But since the climate of ancient Bolivia was mainly tropical, she concluded this was unlikely.

Christian de Muizon, a palaeontologist at the Natural History Museum in Paris and a co-author of the paper, has another interpretation. "Marsupials were at this time radiating across South America. Gregarious behaviour may have made settlement in new regions and in new environments easier, favouring reproduction and parental care. Once populations became established, the gregariousness no longer proved as useful and was lost over time. Of course, we need many more fossil finds to prove this correct." 

• References

  1. Ladevèze, S. , de Muizon, C. , Beck, R. M. D. , Germain, D. &Cespedes-Paz, R. Nature doi:10.1038/nature09987 (2011).