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Electric and magnetic fields and health

index/glossary | EMFs At A Glance | EMF The Facts (pdf)
  • What are EMFs
    • Terminology – an introduction
    • Electric fields
    • Magnetic fields
    • Units for measuring EMFs
    • Measuring and calculating EMFs
      • “EMF Commercial”
    • Adding fields together
    • Radiofrequencies
    • Screening EMFs
  • Sources
    • Overhead power lines
      • Fields from specific power lines
        • 400 kV
        • 400 kV – specific cases
        • 275 kV
        • 132 kV
        • 66 kV
        • 33 kV
        • 11 kV
        • 400 V/230 V
        • Replacing a 132 kV line with a 400 kV line
      • Summaries of fields from all power lines
      • Factors affecting the field from a power line
        • Voltage
        • Current
        • Clearance
        • Height above ground
        • Conductor bundle
        • Phasing
        • Balance between circuits
        • Balance within circuit
        • Ground resistivity
        • Two parallel lines
      • Calculating and measuring fields from power lines
        • Geometries of power lines
        • Raw data
        • On-line calculator
      • Fields from power lines – more detail on the physics
        • Field lines from a power line
        • The direction of the field from a power line
        • Power law variations in the field from a power line
      • Statistics of power line fields
    • Underground power cables
      • Different types of underground cable
      • Fields from cables in tunnels
      • Gas Insulated Lines (GIL)
      • Underground cables with multiple conductors
      • Effect of height on fields from underground cables
      • Screening fields from underground cables
    • Low-voltage distribution
      • UK distribution wiring
      • USA distribution wiring
    • House wiring
    • Substations
      • National Grid substations
        • Static Var Compensators
      • Sealing-end compounds
      • Distribution substations
      • Final distribution substations
        • Indoor substations
    • Transport
      • EMFs from electric trains (UK)
      • EMFs from cars
    • Appliances
    • Electricity meters
      • Smart meters
      • Traditional meters
    • Occupational exposures
      • Live-line work
      • Static Var Compensators
      • Occupational exposures on pylons
    • Field levels and exposures
      • Personal exposure
      • Other factors that vary with magnetic fields
      • Fields greater than 0.2 or 0.4 µT
    • Screening EMFs
      • Screening fields from underground cables
      • EMF Reduction Devices
  • Known effects
    • Induced currents and fields
    • Microshocks
      • Control of microshocks in the UK
      • Microshocks from bicycles
      • Bees and microshocks
    • EMFs and medical devices
      • Standards relating to pacemakers and other AIMDs
    • Effects of EMFs on equipment
  • Research
    • Types of research
    • Epidemiology
    • Animal and laboratory experiments
    • Mechanisms
    • Specific studies
      • UKCCS
      • CCRG
      • French Geocap study
      • CEGB cohort
      • Imperial College study
  • Current evidence on health
    • Childhood leukaemia
      • Survival from childhood leukaemia
      • Childhood leukaemia and Downs
      • Childhood leukaemia and night-time exposure
      • The “contact current” hypothesis
    • Other health effects
    • Scientific review bodies
      • WHO
      • IARC
    • Electric fields and ions
    • Comparing EMFs to other issues
  • Exposure limits for people
    • Limits in the UK
    • Limits in the EU
    • Limits in the USA
    • Limits in the rest of the world
    • Limits from specific organisations
      • ICNIRP 1998
      • ICNIRP 2010
      • NRPB 1993
      • NRPB 2004
      • EU 2004
      • EU 2013
  • Policy
    • UK policy
      • Power lines and property – UK
    • Compliance with exposure limits
    • European EMF policy
    • Precaution
    • SAGE
      • SAGE First Interim Assessment
        • Government response to SAGE First Interim Assessment
      • SAGE Second Interim Assessment
        • Government response to SAGE Second Interim Assessment
        • SAGE Second Interim Assessment – the full list of recommendations
  • Finding out more
    • EMF measurement and commercial services
    • Links
    • Literature
    • Contact us
  • Static fields
    • Static fields – the expert view
You are here: Home / Research / Epidemiology / Causation – what can epidemiology show and what can’t it?

Causation – what can epidemiology show and what can’t it?

On another page we give a simple tutorial on how epidemiology works.  Here, we address the question: what can you deduce from an epidemiological study? What does it show? And how do epidemiologists go about deciding whether it demonstrates causation or not?

What can you deduce from an epidemiological study?

Consider an epidemiological study of EMFs and childhood cancer.

The study looks at a certain number of specific children (for example it might look at 200 children with cancer and compare them to 200 children without cancer). Suppose it finds a statistical association between exposure to EMFs and cancer:

So we know there is a statistical association in the study.

One reason for this might just be chance. Perhaps the 200 children, just by chance, included more of the children exposed to EMFs. If we chose a different 200 children, we wouldn’t find the association. The association in our study does not reflect a true association in the population.

Another reason might be bias. Maybe the way we chose the children meant we got more children with cancer exposed to EMFs, or we got fewer comparison children exposed to EMFs. Or maybe there was some systematic bias in the way we measured exposure. So the children in our study are not representative of the whole population, and the association in our study does not exist in the population. More on bias in studies of EMFs.

But if neither of these applies, then:

We know there is a statistical association in the population.

This still might not mean that EMFs cause cancer. There might be something else in the environment – a factor X. Suppose X causes childhood cancer. But suppose that the children who are exposed to X are also exposed to EMFs. Then it would look as if cancer was associated with EMFs, but this would be a side effect of X. This is called “counfounding” and X is a “confounding factor”.  More on confounding factors.

But if there is no confounding, then:

We know there is a causal effect

Epidemiologists tend to approach a result in the order we’ve laid it out here. They tend to look at a study which has found an association, and accept it as establishing causation only if the alternatives – chance, bias, confounding – don’t seem likely.  See below for the factors epidemiologists take into account.

Two things to note: some of these effects can work in either direction. A bias in the measurements, for example, could often mean that the study finds a smaller association than there really is. And there doesn’t have to be just one of these in any one study, you could have some of the association explained by chance and some by bias, for example.

How epidemiologists decide on causation

In our introduction to epidemiology we explain how an observation of a statistical association between an exposure and a disease may be evidence of causation, or it may have other explanations, such as chance, bias or confounding.

Deciding whether to deduce causation or not is a judgement.  But there are yardsticks to help with that judgement.  A statistical association observed in an epidemiological study is more likely to be causal if:

  • it is strong (the relative risk is reasonably large)
  • it is statistically significant
  • there is a dose-response relationship - higher exposure seems to produce more disease
  • the scope for bias or confounding seems limited
  • the evidence comes from more than one study, preferably studies of different design
  • there is biological support from laboratory experiments
  • it is physically plausible

The Bradford-Hill criteria

One often-quoted attempt to set out these factors was by Sir Austin Bradford Hill.  His list was:

  • Strength
  • Consistency
  • Specificity
  • Temporal relationship
  • Biological gradient
  • Plausibility 
  • Coherence
  • Experiment
  • Analogy

People sometimes try to use these criteria as a ticklist - tick them all and you've proved causation.  But Hill himself was clear these are just a guide and cannot take away the need for an informed judgement in each specific case.

The IARC rules

One organisation which has developed rules for assessing the strength of evidence and whether it amounts to evidence of causation or not is the International Agency for Research on Cancer.  See more on their rules.

See also:

  • How epidemiology fits with other types of research
  • The epidemiology of EMFs and childhood cancer in particular
  • Three of the key epidemiological studies: the UKCCS, the Ahlbom pooled analysis, and the Draper study

Latest news

  • New publication on cancer incidence from the UK electricity industry Cohort Study August 27, 2019
  • How has the reported risk for childhood leukaemia changed over time? February 11, 2019
  • Media stories about microshocks in children’s playground September 10, 2018
  • New studies on leukaemia and distance from power lines June 1, 2018
older news

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To contact the electricity industry’s EMF Unit Public Information Line (UK only):
telephone 0845 7023270 or email [email protected].

See Contact us for more contact details including our privacy policy.

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Navigation
  • What are EMFs
    • Terminology – an introduction
    • Electric fields
    • Magnetic fields
    • Units for measuring EMFs
    • Measuring and calculating EMFs
      • “EMF Commercial”
    • Adding fields together
    • Radiofrequencies
    • Screening EMFs
  • Sources
    • Overhead power lines
      • Fields from specific power lines
        • 400 kV
        • 400 kV – specific cases
        • 275 kV
        • 132 kV
        • 66 kV
        • 33 kV
        • 11 kV
        • 400 V/230 V
        • Replacing a 132 kV line with a 400 kV line
      • Summaries of fields from all power lines
      • Factors affecting the field from a power line
        • Voltage
        • Current
        • Clearance
        • Height above ground
        • Conductor bundle
        • Phasing
        • Balance between circuits
        • Balance within circuit
        • Ground resistivity
        • Two parallel lines
      • Calculating and measuring fields from power lines
        • Geometries of power lines
        • Raw data
        • On-line calculator
      • Fields from power lines – more detail on the physics
        • Field lines from a power line
        • The direction of the field from a power line
        • Power law variations in the field from a power line
      • Statistics of power line fields
    • Underground power cables
      • Different types of underground cable
      • Fields from cables in tunnels
      • Gas Insulated Lines (GIL)
      • Underground cables with multiple conductors
      • Effect of height on fields from underground cables
      • Screening fields from underground cables
    • Low-voltage distribution
      • UK distribution wiring
      • USA distribution wiring
    • House wiring
    • Substations
      • National Grid substations
        • Static Var Compensators
      • Sealing-end compounds
      • Distribution substations
      • Final distribution substations
        • Indoor substations
    • Transport
      • EMFs from electric trains (UK)
      • EMFs from cars
    • Appliances
    • Electricity meters
      • Smart meters
      • Traditional meters
    • Occupational exposures
      • Live-line work
      • Static Var Compensators
      • Occupational exposures on pylons
    • Field levels and exposures
      • Personal exposure
      • Other factors that vary with magnetic fields
      • Fields greater than 0.2 or 0.4 µT
    • Screening EMFs
      • Screening fields from underground cables
      • EMF Reduction Devices
  • Known effects
    • Induced currents and fields
    • Microshocks
      • Control of microshocks in the UK
      • Microshocks from bicycles
      • Bees and microshocks
    • EMFs and medical devices
      • Standards relating to pacemakers and other AIMDs
    • Effects of EMFs on equipment
  • Research
    • Types of research
    • Epidemiology
    • Animal and laboratory experiments
    • Mechanisms
    • Specific studies
      • UKCCS
      • CCRG
      • French Geocap study
      • CEGB cohort
      • Imperial College study
  • Current evidence on health
    • Childhood leukaemia
      • Survival from childhood leukaemia
      • Childhood leukaemia and Downs
      • Childhood leukaemia and night-time exposure
      • The “contact current” hypothesis
    • Other health effects
    • Scientific review bodies
      • WHO
      • IARC
    • Electric fields and ions
    • Comparing EMFs to other issues
  • Exposure limits for people
    • Limits in the UK
    • Limits in the EU
    • Limits in the USA
    • Limits in the rest of the world
    • Limits from specific organisations
      • ICNIRP 1998
      • ICNIRP 2010
      • NRPB 1993
      • NRPB 2004
      • EU 2004
      • EU 2013
  • Policy
    • UK policy
      • Power lines and property – UK
    • Compliance with exposure limits
    • European EMF policy
    • Precaution
    • SAGE
      • SAGE First Interim Assessment
        • Government response to SAGE First Interim Assessment
      • SAGE Second Interim Assessment
        • Government response to SAGE Second Interim Assessment
        • SAGE Second Interim Assessment – the full list of recommendations
  • Finding out more
    • EMF measurement and commercial services
    • Links
    • Literature
    • Contact us
  • Static fields
    • Static fields – the expert view