Friday November 7 2008
“Green spaces reduce [the] health gap between rich and poor”, The Independent says today. It reports that the inequality in health between the rich and the poor can be halved with the help of green spaces. The studyon which the news story was based looked at the entire population in England under retirement age and found that the greatest effect was in circulatory diseases, while there were no apparent benefits of green space on deaths from lung cancer.
The study found that the amount of green space in the few kilometres around where people live affects the size of the ‘gap’ in health inequality between the most and least deprived people in that area. While the study design cannot prove that green space redcues health inequalities, the implication of this study is a positive one and should be endorsed.
Where did the story come from?
Dr Richard Mitchell and Frank Popham from the University of Glasgow and the University of St. Andrews carried out this study. The authors report that their study has no direct sponsor. The study was published in the peer-reviewed medical journal: the Lancet.
What kind of scientific study was this?
The researchers say that exposure to green space, or “the natural environment”, has an effect on people’s health and on their “health related” behaviours. There is also a known relationship between health and income, with people who are better off tending to be healthier. Their theory was that this inequality in health between higher and lower income groups, would be less pronounced in areas with more green space.
The study is in essence a cross-sectional analysis. The authors looked at the English populations’ exposure to green space and measures of income inequality and then obtained individual data on the deaths occurring within the areas. Green spaces in this context are defined as ‘open, undeveloped land with natural vegetation’ and included parks, forests, woodlands, playing fields etc.
The researchers assessed the amount of green space per lower level super output area (LSOA) – a small geographical area used by the Office for National Statistics. LSOAs have a minimum population of 1,000 people and an average area of 4 square kilometres. Green space data was available from the generalised land use database published by the government. The researchers looked at the entire of England and characterised people’s exposure to green space into five categories. The categories ranged from one (least exposed) to five (most exposed). Each category therefore contained 20% of the English population.
Individual death records (from the LSOA of residence) were found for deaths between 2001 and 2005 from the UK Office for National Statistics. These gave cause of death, age at death and gender, but the individuals were anonymous. The researchers excluded men and women older than retirement age (60 years for women, 65 years for men) because ‘inequalities in mortality tend to be at a maximum in the working-age population’. They focussed on ‘all-cause’ mortality (deaths from all causes), and also looked specifically at deaths from circulatory diseases, deaths from lung cancer and deaths from intentional self-harm.
To determine the ‘income’ of people per area, the researchers used a measure known as the English Index of Multiple Deprivation and grouped LSOAs into four groups, ranging from least deprived to most deprived.
The researchers then used statistical analysis to see if there was a link between exposure to green space and income deprivation, exposure to green space and mortality and whether the association between income deprivation and mortality varied depending on the amount of green space in the geographical area of residence. In their analyses, they adjusted for other factors that may be playing a part in this relationship, including age group, sex, education, skills and training, living environment, population density and whether the area was urban or rural.
What were the results of the study?
People with more exposure to green space were less likely to be deprived than those with little exposure. There was also an independent link between exposure to green space and all-cause mortality. This link was also apparent for deaths from circulatory diseases, but not for deaths from lung cancers or from intentional self-harm.
The researchers also found that the link between income deprivation and mortality (all-cause and from circulatory disease) varied according to the green space exposure. In other words, there was greater health inequality between high and low income groups in areas with little green space than for those in areas with a lot of green space. The researchers estimated that the reduced health inequality in areas with more green space saved approximately 1,328 lives per year.
What interpretations did the researchers draw from these results?
The researchers conclude that inequality in all-cause and circulatory disease mortality due to income deprivation is lower in people who live in regions of high green space compared to people living in low green space areas. They conclude that this reduction in inequality may be due to the fact that green space encourages physical activity and reduces stress.
What does the NHS Knowledge Service make of this study?
The authors acknowledge some weaknesses of their study:
- Firstly they were not able to determine whether the people living in the area actually had access to the green space, nor were they able to determine the quality of that green space.
- As they were only looking at people’s exposure to green space at the time of their death, they could not know how long people had lived in the area and what an individual’s exposure to green space had been during their lifetime. It is possible that some people migrated to a different area because of their illness.
- Given that exposure to green space was also strongly associated with high incomes there are a number of other factors that might have been responsible for the differences in inequality seen here. These include access to healthcare and factors such as an individual’s smoking status, diet, physical activity, genetics, mental health etc. While the researchers attempted to adjust for some of the important factors (e.g. air pollution, education), they made the assumption that areas have the same exposure to ‘welfare state’ and ‘health service’. This may not be true for individuals.
The researchers say that the idea that different types of physical environment might have an effect on health inequalities is a novel one and that changing the environment in which people live is most likely to affect inequalities at the population level.
The inherent weaknesses of the study mean that it is not possible to conclusively say that exposure to green space is responsible for the observed reduction in health inequalities.
Sir Muir Gray adds...
Everyone needs a Natural Health Service as well as a National Health Service.