Wednesday February 13 2008
Noise during the night can disturb sleep
“Aircraft noise can kill, report claims”, is the headline in The Daily Telegraph. The article goes on to say that “thousands of people are killed each year by increases in blood pressure triggered by night-time noise from aircraft”. The Independent says, “the effect of noise on sleep has found that a snoring partner can raise a sleeper’s blood pressure by as much as a low-flying aircraft”. Other news sources report either that aircraft noise or snoring is bad for your health.
The news reports are based on a study by scientists who monitored 140 volunteers in their homes near Heathrow and three other European airports to see how their blood pressure responded to noise as they slept. The researchers concluded that the noise in the bedroom from aircraft had the same effect on the volunteers as noises coming from inside the same room, e.g. snoring. However, the study only looked at changes in blood pressure, and any increase was temporary. It is unlikely that these noises would lead to death.
Where did the story come from?
Dr Alexandros Haralabidis and members of the HYENA (Hypertension and Exposure to Noise near Airports) Consortium carried out this research. HYENA is a research group with affiliations with academic and health institutes in Greece, Italy, UK, Sweden, the Netherlands, and Germany. The study was funded by a grant from the European Commission. It was published in the peer-reviewed medical journal: European Heart Journal.
What kind of scientific study was this?
This was an observational cross-sectional study that recorded the blood pressure in selected volunteers living close to airports in four European cities: Athens (Greece), Malpensa (Italy), Arlanda (Sweden) and London Heathrow (UK). For one night each, the researchers attached ambulatory blood pressure monitors (ABPM) to 140 volunteers in the four countries (16 volunteers lived near Heathrow). These devices record blood pressure by inflating and deflating every 15 minutes throughout the night and about 30 blood pressure measurements are taken during sleeping hours. The researchers also measured the participants’ heart rate.
The participants were on average about 50 years old and had sleeping blood pressures of about an average 110/65 mmHg (taken before the recording started), which is considered normal. During the study night, the researchers recorded noises with an MP3 player so they could identify the source, and measured the noise levels in the bedroom with a connected noise meter. They calculated both an average background level of noise and the maximum level of noise for the 15 minutes between blood pressure measurements.
They used statistical models to analyse the changes in blood pressure of the group as a whole and related this to the average noise levels between blood pressure measurements and the maximum noise levels measured just before blood pressure measurements, as well as noise events. A noise (airport, road traffic, or indoor) had to reach over 35 decibels before being classified as an ‘event’.
What were the results of the study?
Most of the aircraft noise events recorded at above a maximum of 35 decibels occurred in Athens, where the average number of events during the night was 19. The average number of aircraft events in London was zero. The average for Milan was two and for Stockholm, zero. The number of road traffic events was also low, less than one on average across all four cities. Indoor noise events, such as snoring, occurred on average between five and 14 times.
The researchers say that an increase of 6.2 mmHg in systolic blood pressure and 7.4 mmHg in diastolic blood pressure was observed over the 15-minute interval in which an aircraft event occurred; there were less consistent effects observed on heart rate. When the actual maximum noise level of an event was considered, there were no differences in the effect on blood pressure according to the noise source.
What interpretations did the researchers draw from these results?
The researchers claim the “effects of noise exposure on elevated subsequent blood pressure measurements were clearly shown”. They also say that the effect of the noise level is not related to the noise source.
What does the NHS Knowledge Service make of this study?
There are some surprising conclusions from this study and its reporting. The researchers acknowledge some of the limitations to the design and some issues with the measurement of blood pressure in this study. These include:
- Snoring was only one of the indoor noise events that were recorded, and the researchers were unable to determine if it was the subject or their partner who was snoring. Snoring may increase an individual’s heart rate and blood pressure through mechanisms other than noise. None of these results can be reliably interpreted as the effect of a partner snoring.
- The average (pooled) increases in blood pressure that were shown in this study were less than 1 mmHg for every five decibels of noise, measured over one or 15 minutes before blood pressure measurement. The main result quoted from this study – the 6–7 mmHg difference in blood pressure – is based on the pooling of a relatively small number of blood pressure increases.
- Aircraft events were much more commonly heard in the homes of the 43 participants in Athens, who were more likely to be women. The differences in the recordings from the four sites suggests it may not be appropriate to combine all the events in a pooled analysis.
- Only a small number of aircraft events (less than 10%) occurred in the exact minute that blood pressure was being measured.
This study provides some data to support calls for greater protection from noise, but the links between noise and death from heart disease, claimed by some news sources, seem overstated.