Hot weather headache

A new study suggests that “hot weather can trigger migraines and other debilitating types of head pain”, reported The Daily Telegraph . The newspaper said the research also found that a drop in air pressure can increase the risk of a headache. The study reportedly looked at 7,054 people who attended casualty with severe head pain, and examined whether the weather conditions in the past three days was linked to the frequency of these headaches. It found that an increase of 5ºC raised the risk of a severe headache within 24 hours by 7.5%.

This relatively large study suggests that weather conditions can affect the risk of headache. However, it is worth noting that the effect was relatively small, and may be caused by factors other than temperature. For example, there was no information on the sufferers’ activity levels, stress and emotional levels, or food and drinks they had consumed. All of these factors can affect the risk of migraine in some people. Also, the study only assessed headaches presenting to hospital, so the findings cannot be applied to people who manage their headaches at home.

Where did the story come from?

Dr Kenneth J. Mukamal and colleagues from the Beth Israel Deaconess Medical Center and the Harvard School of Public Health in Boston carried out this research. The work was supported by grants from the National Institute of Environmental Health Sciences and the US Environmental Protection Agency. The study was published in the peer-reviewed medical journal Neurology .

What kind of scientific study was this?

This was a case-crossover study that looked at the effects of weather conditions and air pollution on the risk of developing severe headaches.

The researchers identified all people who had attended the casualty department between May 2000 and December 2007 with a diagnosis of headache (including migraine and tension headache). People who lived more than 40km from the hospital were excluded because the weather and air pollution may have differed from conditions closer to the hospital. This left 7,054 people (average age 42 years; 75% women) for inclusion in the study. Of these, 4,803 had tension or unspecified headaches, and 2,250 had migraines.

The researchers looked at the weather and air pollution conditions for the three days before each person attended casualty. They compared these conditions with those for three control days in the same month. To select control days, the researchers identified the day of the week on which the person presented to casualty, and took all other occurrences of that day in the month as control days (that is, if the person presented at casualty on a Wednesday in May, all of the other Wednesdays in May acted as the control days).

Data on weather and pollution were obtained from local meteorological and pollution-monitoring stations. The researchers used statistical methods to examine the effects of incremental increases in temperature, barometric pressure, and air pollutants (nitrogen and sulphur dioxide, black carbon and fine particulate matter). They also looked at the time lag between changes in these factors and the time the person went to casualty for the headache (between 0 to 72 hours).

What were the results of the study?

The researchers found that higher temperatures increased the odds of presenting at hospital with headache within the next 24 hours. For a 5ºC increase in temperature, the odds of presenting to casualty with a headache within the next 24 hours increased by 7.5%.

Air pressure did not have a significant effect on the overall risk of presenting to casualty with a headache, but it did affect the risk of presenting with non-migraine headache. Lower air pressure increased the risk of developing a non-migraine headache within 48 to 72 hours. A reduction in air pressure of 5mmHg was associated with roughly a 6% reduction in the odds of presenting to casualty with a non-migraine headache.

The levels of air pollutants did not significantly affect the risk of overall headache or of specific types of headache.

What interpretations did the researchers draw from these results?

The researchers concluded that increasing temperature and, to a lesser extent, lower air pressure temporarily increase the risk of presenting to casualty with severe headache. They also say that although no link was found between air pollutants and headache risk, there could still be a link but this study was not large enough to detect it.

What does the NHS Knowledge Service make of this study?

This study has shown a link between the weather and the risk of presenting to casualty with headache. However, there are some points and limitations to note:

• It is a relatively small increase in the risk of headache.
• There is the possibility that factors other than the weather affected the results. For example, there was no information available on an individual’s level of activities on the day that their headache occurred, nor their stress and emotional levels, or food and drink consumption, etc. All of these can affect the risk of a headache in certain individuals.
• The study assessed any headaches presenting to hospital. As such, these findings cannot be applied to the large number of people who manage their headaches at home without going to hospital.
• The type of headache presenting at hospital could have been misdiagnosed because diagnosing the exact type of headache can be difficult. For example, some people who suffer from migraines do not suffer from some of its classic symptoms, such as visual aura. Misclassification of the three types of headache may have lead to inaccuracies when the subanalysis was carried out according to the type of headache.
• Measurements of weather conditions and air pollution were taken at central locations and may not have accurately reflected each individual’s exposure.
• It is possible that temperature and air pressure affect the severity of headaches rather than the risk of having a headache. This would mean the increased numbers of people presenting to hospital with headaches was the result of the headaches being more severe rather than more people having headaches.