Researchers have made an important first step in assessing a novel method of comparing population-based health data with environmental conditions. The longitudinal-ecological study looked at the link between mortality from two causes and the presence of the Emerald Ash Borer beetle.
Donovan GH, Butry DT, Michael YL, et al. The relationship between trees and human health: evidence from the spread of the emerald ash borer. Am J Prev Med. 2013;44(2):139-145.
A longitudinal-ecological study (ie, observing population-based changes over time) analyzing data from a natural experiment. Assessment of lower-respiratory tract disease (LRTD) and cardiovascular disease (CVD) mortality was conducted to determine association with the presence of the Emerald Ash Borer beetle (Agrilus planipennis) within select US counties, via longitudinal regression models.
Residents of any of the 15 US states exposed to A. planipennis during the study period (1990–2007), taken from US Census data. Demographic data (eg, socio-economic status, race/ethnicity) were included as covariates.
Exposure was determined based on history of residence in a US state impacted by the presence of A. planipennis between 1990 and 2007. A. planipennis is a parasitic beetle that inhabits all known species of ash tree (Fraxinus spp.), with infestation resulting in tree death and substantial decline of ash trees from the local floral diversity. US infestation began in 2002 in Michigan and has spread to 15 US states in the Great Lakes/Mid-West region as of 2007 (last data available). Presence of A. planipennis in a county is not a direct determinant of health itself—it's not toxic to humans, is not an infectious disease vector, etc—but it is used as a proxy measure of Fraxinus tree loss. Longitudinal changes in Fraxinus prevalence and presence of A. planipennis (by county) were used as variables in regression modeling.
Mortality data from LRTD, CVD, and accidental death (ie, control) were obtained from the National Center for Health Statistics. Data were available as county-aggregate mortality rates and were collected for both the pre-A. planipennis (1990–2001) and post-A. planipennis (2002–2007) periods. Linear time-trend variables were included in all regression models to account for broad temporal trends in mortality unrelated to the study variables (eg, improved medical technology).
Presence of A. planipennis was associated with time-lag increases in both LRTD and CVD mortality within county of residence (after controlling for potential confounders). An additional 6.8 LRTD deaths (95% CI: 4.8, 8.7; P<0.001) and 16.7 CVD deaths (95% CI: 5.7, 27.7; P=0.001) per year per 100,000 adults were revealed during the exposure period 2002–2007 in infested counties.* These associations significantly increased over time such that there was a positive marginal duration-of-infestation effect. It was calculated that an additional 6,113 LRTD deaths and 15,080 CVD deaths occurred during this 5-year period associated with A. planipennis infestation.
Analysis of accidental death rates via the same regression models showed no additional deaths during the 2002–2007 period. This was the anticipated result, as there is no plausible explanatory mechanism linking A. planipennis beetle presence and accidental death rate. Analysis was conducted as a control-check on the validity of the longitudinal regression model.
*For reference: Total mortality rate in the US is approximately 750 deaths per year per 100,000 adults, with 40.2 chronic LRTD deaths (excluding asthma) and 193.6 CVD deaths per year per 100,000 adults.1
This study is a unique example of the field of research connecting “nature” (ie, the natural environment) to human health. Since the first empirical study on this topic was published in Science almost 30 years ago,2 a broad range of evidence has confirmed one of the basic tenets of naturopathic medicine: that there is an inherent healing power of nature, an intrinsic connection between the natural world and human health and well-being.3 Multiple studies have detected a statistical relationship between green space and morbidity4 and mortality,5 as well as other health-related outcomes such as birth weight and head circumference.6,7 This evidence has been so compelling that the American Academy of Pediatrics has spoken out about the need for people to spend more time in the outdoors,8 and the American Public Health Association is soon adopting a new policy statement on Nature, Health, and Wellness.
However, this current study is the first to investigate the effects of rapid ecosystem changes on health outcomes. This type of experiment is only possible as a result of Fraxinus ash tree decimation from unwanted A. planipennis beetle infestation in the eastern US region. A total count of lost ash trees would be very laborious, but use of beetle presence as a proxy measure can be conducted quite easily. The lack of health risk from exposure to the Emerald Ash beetle itself, in conjunction with inclusion of potential confounders in regression models, demonstrates (according to the study authors) that any associated mortality can be attributed to Fraxinus loss in the population studied.
In the literature on this topic, there are a number of possible contributing factors to explain this relationship between tree loss and human mortality rate:
- Vegetation benefits overall outdoor air quality9 via filtration of particulate matter,10 VOCs,11 and other airborne pollutants. Reduction of air pollution–related morbidity and mortality via vegetation filtration has been successfully demonstrated in both integrated modeling12 and previous ecological studies.13 A decline in ash tree population could be reasonably associated with worsening air-quality related to LRTD and CVD.
- The presence of green space has beneficial modulatory effects on levels of perceived and experienced stress.14 Individual and population-based studies have demonstrated that natural environments positively affect objective measures of stress such as heart-rate variability and salivary cortisol.15,16 Reduction of psychophysiological stress directly decreases allostatic load, the deleterious accumulation of physiological processes (such as inflammation and immune-suppression) that leads to LRTD and CVD.17 Loss of ash tree vegetation could potentially limit stress-reduction experiences and result in elevated allostatic load.
- A related effect might be the psychological impact of environmental disruption as discussed by professional ecopsychologists.18 Terms such as “place attachment,” “ecoanxiety,” and “solastalgia”** reflect the increasing reality of our era, when massive environmental habitat loss is both unavoidable and uncontrollable at the individual level. Relationship between these ecopsychological concepts and organic pathophysiological processes is purely speculative at this time, and no direct mechanisms have been empirically tested. However, it is conceivable that the stresses induced by concern over climate change are similar to other life stresses regarding their ability to influence morbidity and mortality.
- Lastly, outdoor vegetation is a recognized incentive for promotion of healthy behaviors such as physical activity and social interaction,19–21 though it is unlikely that decline in Fraxinus prevalence was associated with decreases in these behaviors.
Given the ecological nature of the study, direct causal attribution of individual mortality to tree loss is not possible. This current paper was an exploratory first step in assessing a novel method of comparing population-based health data with environmental conditions. More work will be necessary to determine how ecological changes like Fraxinus decimation affect health on a personal level.
Still, this study does have implications for the clinical practitioner. First, practitioners in counties with A. planipennis infestation may investigate how the changing vegetation landscape is affecting the health of their patients and themselves. More broadly, this study identifies ecosystem forces as potential modulators of mortality and calls attention to the multiple ways environmental factors may be influencing health and well-being. Research of this kind helps expand the concept of “environmental health” beyond the typical toxicological model and suggests consideration of a larger and more complex systems-based model of nature in which we live.
It is important to note that there were some limitations of this study (ie, ecological design; degree of beetle infestation not measured; Fraxinus prevalence imputed), and that not all research in this area has demonstrated significant associations between green space exposure and health measures.22,23 In contrast, the number of supportive studies in this research area suggest the study results are valid.
** Solastalgia: “the distress that is produced by environmental change impacting on people while they are directly connected to their home environment.”24