Tuberculosis mortality is influenced by the density of residential trees

Relation

Blount RJ, Pascopella L, Barry P, et al. Canopy cover of urban residential trees is associated with reduced mortality during tuberculosis treatment in California.Sci Total Environ. 2020;711:134580.

Study objective

To determine the association between urban tree cover and mortality from community-acquired tuberculosis (TB) infection.

Participant

All patients diagnosed with TB infection in the state of California between the years 2000 and 2012 (N = 33,962; mean age, 46.9 years; 59.4% male), according to the California TB Registry.

Draft

Researchers tracked participants' morbidity and mortality status and compared it to the amount of tree cover surrounding their home 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 nationwide satellite-based vegetation survey database. The researchers used 4 different buffer zones with different radii (50 m, 100 m, 200 m and 300 m), with tree cover represented as a percentage of total land area and divided into quintiles for analysis.

Data were controlled for demographic, socioeconomic, and clinical covariates: age, gender, race, ethnicity, immigration within 1 year, household income, employment status, substance abuse, homelessness, and concurrent HIV infection.

Target parameters

Researchers assessed 2 outcome measures:

• Sterblichkeitsrate, unterteilt in 3 Kategorien: TB-bedingt, kardiopulmonal und andere)
• Acid-fast Bacilli (AFB) Sputum-Abstrich: Die übliche Methode zur Bestimmung des Vorhandenseins von pathogenen TB-Bakterien

Key insights

For each of the 4 buffer radii (50 m-300 m), there was an inverse relationship between percent tree cover and all-cause, TB-related, and cardiopulmonary mortality. After adjustments for all covariates, comparing the highest to lowest tree cover quintiles showed significant reductions in all-cause mortality risk as follows:

• 50-m-Puffer: 22 % reduziertes Risiko (HR 0,78, 95 % KI 0,68–0,90, P=0,0003)
• 100-m-Puffer: 21 % reduziertes Risiko (HR 0,79, 95 % KI 0,68–0,91, P=0,002)
• 200-m-Puffer: 13 % reduziertes Risiko (HR 0,87, 95 % KI 0,75–1,00, P=0,02)
• 300-m-Puffer: 11 % reduziertes Risiko (HR 0,89, 95 % KI 0,77–1,04, P=0,03)

Additionally, the researchers measured an inverse relationship between percent tree cover and AFB sputum swab positivity for each of the 4 buffer radii. After adjustments for all covariates, comparing the highest to lowest tree cover quintiles showed significant reductions in AFB sputum smear positivity as follows:

• 50-m-Puffer: 7 % reduzierte Chancen (OR 0,93, 95 % KI 0,86–1,01, P=0,02)
• 100-m-Puffer: 11 % reduzierte Chancen (OR 0,89, 95 % KI 0,82–0,96, P=0,001)
• 200-m-Puffer: 14 % reduzierte Chancen (OR 0,86, 95 % KI 0,79–0,93, P=0,0002)
• 300-m-Puffer: 12 % reduziertes Risiko (OR 0,88, 95 % KI 0,81–0,96, P=0,002)

AFB sputum smear positivity was directly associated with patient mortality, as is typical of TB infection.

comment

This current study is the first to examine the effects of green spaces on a specific respiratory infectious disease (TB). Readers of the literature “Green Space and Health” are aware that living proximity to various forms of “nature” has a positive effect on health status and the prevalence of diseases.¹Large-scale public health surveys have shown the inverse relationship between living near green spaces and mortality rates.^2.3In particular, a recent study from China showed that mortality rates from respiratory diseases decrease when people live surrounded by more dense greenery.⁴

This is a timely finding to potentially address the current global COVID-19 pandemic, which has also shown an association between air pollution concentrations and respiratory morbidity and mortality.

There are many mechanisms responsible for nature's health-promoting ability that have been discussed in this and other journals.^5.6One of the most relevant factors associated with mortality from respiratory diseases is the ability of vegetation to reduce air pollution and improve air quality. This happens via 2 processes:

1. Das Ablage von Feinstaub (PM), wie Ruß, Asche und Verbrennungsprodukten, auf bewachsenen Oberflächen, anstrengen sie aus der Luft; und
2. Das Absorption von toxischen Gasen (z. B. NO_xÖ₃) in Blätter als Teil der Atmungsfunktion von Pflanzen, filtern sie aus der Luft.

The magnitude of air quality improvement and air pollutant reduction caused by vegetation is significant, with trees estimated to remove approximately 18 million tons of air pollutants annually in the United States.⁷This benefits human health in several ways, including a reduction in pathogenic airborne toxins as well as the removal of immunosuppressive pollutants that promote respiratory disease. These and other benefits (e.g. reduction in air temperature as well as psychophysiological stress and allostatic load^8.9) were calculated to prevent 670,000 cases of acute respiratory illness annually, with an estimated healthcare value of $6.8 billion in savings.¹⁰It is likely that the current study's focus on TB mortality in California is just one of several specific conditions that benefit from improving air quality with greenery.

Other respiratory diseases may similarly benefit from improving air quality near green spaces. This is a timely finding to potentially address the current global COVID-19 pandemic, which has also shown an association between air pollution concentrations and respiratory morbidity and mortality.^11,12Reducing air pollution through vegetative deposition and filtration could help reduce the number of coronavirus cases and deaths. This strategy can be particularly effective in addressing issues of environmental injustice and inequality among the urban poor and communities of color, which have been disproportionately affected by COVID-19, air quality issues, and the lack of healthy green spaces.^13,14

restrictions

This was an observational study and therefore causality of tree density on TB mortality cannot be established. Additionally, the researchers only assessed tree density near the patients' home address, not in other locations such as work or school, where participants may also have encountered green space effects. However, the results of this study combined with other research lend credence to the existence of a strong and statistically significant inverse relationship.

Conclusion

Several factors influence the progression of an infectious disease such as tuberculosis. External factors such as air pollution are known to influence the severity of respiratory diseases. Environmental characteristics, including the density of air quality-enhancing trees surrounding a person's residence, have the potential to significantly influence health status and mortality rates across the population, particularly for at-risk groups in underserved communities.