More green, less asthma
Anew study using ‘‘uniquely rich’’ New Zealand databases has found a strong link between greenness and asthma. ‘‘Exposure to greenness was associated with a significantly lower risk of developing asthma,’’ concluded the paper’s authors, led by Dr Geoffrey Donovan of the US Forest Service in Portland, Oregon.
The authors also found that exposure to a greater number of ‘‘natural land-cover types’’ provided additional protection against allergenic asthma.
In other words, the greater the diversity of plant life children were exposed to, the better when it came to childhood asthma.
The findings were ‘‘suggestive of a link between plant biodiversity and asthma’’ rather than conclusive, the research paper said. This was because not all plants provide the same level of protection. Some plants are known to provoke asthma, for example.
Nonetheless, ‘‘we are one step closer’’ to proving the link, said co-author Jeroen Douwes, professor of public health and director of the Centre for Public Health Research at Massey University.
‘‘But we’re not there yet. To our knowledge, this is the first longitudinal study showing that vegetation diversity can protect against a specific adverse health outcome,’’ the authors wrote in the paper published in the journal Nature Plants in May.
To complete the study, the four authors accessed an amazing New Zealand dataset they described as ‘‘uniquely rich’’ and Douwes described as world-leading.
Statistics New Zealand calls it ‘‘integrated data infrastructure’’, or IDI, and the researchers were given access to about 12 measures of almost 50,000 New Zealand children born in 1998 and tracked since.
The measures – including drug prescriptions, hospitalisations, education, welfare benefits, criminal justice, social deprivation, births, deaths and immigration, income, work and residential history – are drawn from multiple databases and linked by a common ID number for each individual. There were about 166 billion pieces of information in IDI when the paper was written and more now.
The data were anonymised so that no individual can be identified. Addresses are not available, for example, but location is provided by ‘‘meshblock’’, a geographic unit of 30-60 dwellings, or 60 to 120 residents.
This massive data haul – 18 years worth – was then crossreferenced with the ‘‘normalised difference vegetation index’’, which is imagery gathered by satellites and after processing gives a measure of greenness. These data were added at the meshblock level.
Using statistical tools, the researchers found links between greenness, biological diversity and asthma.
They also dug deeper with two species particularly common in New Zealand – gorse and exotic conifers, largely meaning radiata pine. Long childhood exposure to either species slightly increased asthma risk, the researchers found.
The power of the study comes from tracking 49,956 individuals over 18 years, Douwes said. Previous studies tended to measure greenness once in a child’s life, even if they moved.
Greenness derived from satellite imagery is a ‘‘relatively coarse measure’’, the authors wrote. To compensate, they also used LandCare Research’s Land-Cover Database, which categorises all land cover in the country into 35 classes. These were added to the statistical model at the meshblock level.
The study also confirmed previous research that found girls were less likely to have asthma than boys. Asian and Ma¯ ori children had higher rates of asthma than Europeans, but Pacific Island children had lower rates.
Other factors that increased asthma rates included underweight or premature birth, the number of antibiotic prescriptions, a non-smoking mother and a mother who didn’t finish high school.
Factors that protected against asthma included having more siblings and always living in a rural meshblock.
Neither road density nor mean annual nitrogen dioxide concentration seemed to matter, the study found.
‘‘This large longitudinal birth cohort study has shown, for the first time, that exposure to greenness and vegetation diversity may be protective of asthma,’’ the paper concludes.