Blount RJ, Pascopella L, Barry P, et al. Residential urban tree canopy is associated with decreased mortality during tuberculosis treatment in California. Sci Total Environ. 2020;711:134580.
To determine the association between urban tree cover and mortality from community-acquired tuberculosis (TB) infection.
All patients with diagnosed TB infection in the state of California between the years 2000 to 2012 (N=33,962; median age 46.9 years; 59.4% male), according to the California TB registry.
The researchers tracked participants’ morbidity and mortality status and compared it to the amount of tree cover surrounding their residential address (or, in the case of homelessness, the shelter or street intersection closest to where they spend the majority of their time), as determined by a statewide satellite-based vegetation-survey database. The researchers used 4 different buffer zones of varying radii (50 m, 100 m, 200 m, and 300 m), with tree cover presented as a percentage of total land area, divided into quintiles for analysis.
Data were controlled for demographic, socioeconomic, and clinical covariates: age, sex, race, ethnicity, immigration within 1 year, household income, employment status, substance abuse, homelessness, and concurrent HIV infection.
Researchers assessed 2 outcome measures:
- Mortality rate, subdivided into 3 categories: TB-related, cardio-pulmonary, and other)
- Acid-fast Bacilli (AFB) sputum smear: the common method for determining the presence of pathogenic TB bacteria
There was an inverse relationship between percentage of tree cover and all-cause, TB-related, and cardio-pulmonary mortality for each of the 4 buffer radii (50 m–300 m). After adjustments for all covariates, comparison of highest to lowest tree cover quintiles demonstrated significant reductions in all-cause mortality risk as follows:
- 50-m buffer: 22% reduced risk (HR 0.78, 95% CI 0.68–0.90, P=0.0003)
- 100-m buffer: 21% reduced risk (HR 0.79, 95% CI 0.68–0.91, P=0.002)
- 200-m buffer: 13% reduced risk (HR 0.87, 95% CI 0.75–1.00, P=0.02)
- 300-m buffer: 11% reduced risk (HR 0.89, 95% CI 0.77–1.04, P=0.03)
In addition, the researchers measured an inverse relationship between percentage tree cover and AFB sputum smear positivity for each of the 4 buffer radii. After adjustments for all covariates, comparison of highest to lowest tree cover quintiles demonstrated significant reductions in AFB sputum smear positivity as follows:
- 50-m buffer: 7% reduced odds (OR 0.93, 95% CI 0.86–1.01, P=0.02)
- 100-m buffer: 11% reduced odds (OR 0.89, 95% CI 0.82–0.96, P=0.001)
- 200-m buffer: 14% reduced odds (OR 0.86, 95% CI 0.79–0.93, P=0.0002)
- 300-m buffer: 12% reduced risk (OR 0.88, 95% CI 0.81–0.96, P=0.002)
AFB sputum smear positivity was directly associated with patient mortality, as is typical for TB infection.
This current study is the first to investigate the effects of green space on a specific respiratory-based infectious disease (TB). Readers of the “green space and health” literature are aware that residential proximity to various forms of “nature” has a positive impact on health status and disease prevalence.1 Large-scale public-health surveys have shown the inverse association between living in proximity to green space and mortality rates.2,3 Specifically, a recent study from China has demonstrated that rates of respiratory-based mortality decline when people live surrounded by greater density of greenery.4
This is a timely finding for potentially addressing the current global COVID-19 pandemic, which has also shown an association between air pollution concentration and respiratory morbidity and mortality.
There are many mechanisms that account for nature’s health-promoting ability, which have been discussed in this and other journals.5,6 One of the most relevant related to respiratory-disease mortality is vegetation’s ability to reduce air pollution and improve air quality. This occurs via 2 processes:
- The deposition of particulate matter (PM), such as soot, ash, and combustion products, on vegetative surfaces, straining them from the air; and
- The absorption of toxic gases (eg, NOx, O3) into leaves as part of plants’ respiratory function, filtering them from the air.
The magnitude of vegetation-based improvement in air quality and reduction in airborne pollutants is substantial, with 1 estimate calculating around 18 million tons of airborne pollutants removed by trees in the United States annually.7 This benefits human health in multiple ways including a reduction in pathogenic air toxins as well as removal of immuno-suppressive pollutants that facilitate respiratory disease. These and other benefits (eg, reductions in air temperature as well as psychophysiological stress and allostatic load8,9) have been calculated to prevent 670,000 cases of acute respiratory disease annually, with an assessed healthcare value of $6.8 billion saved.10 It is likely the current study’s focus on TB mortality in California is just 1 of multiple specific conditions benefitted by greenery-based air-quality improvements.
Other respiratory conditions may similarly benefit from proximity to green spaces’ air-quality improvements. This is a timely finding for potentially addressing the current global COVID-19 pandemic, which has also shown an association between air pollution concentration and respiratory morbidity and mortality.11,12 Reductions in ambient air pollution via vegetative deposition and filtration could assist in decreasing coronavirus cases and deaths. This strategy may be especially effective in addressing issues of environmental injustice and inequality among communities of the urban poor and communities of color disproportionally affected by COVID-19, air-quality issues, and lack of healthy green spaces.13,14
This was an observational study, and therefore, causality of tree density on TB mortality cannot be established. In addition, the researchers assessed tree density only around patients’ residential address, not other locations such as work or school where participants may also have encountered green space effects. However, the findings of this study in conjunction with other research lend credibility to the existence of a strong and statistically significant inverse relationship.
Multiple factors influence the progression of an infectious disease like TB. External factors such as air pollution are well-known to modulate respiratory-disease severity. Environmental features, including the density of air quality–improving trees surrounding an individual’s residence, have the potential to significantly affect health status and mortality rates throughout the population, particularly for at-risk groups in underserved communities.