Vipin Kumar and Sonali Chauhan

There is an increasing demand for residential and commercial spaces , as infrastructure is often preferred over natural spaces in the city. The industrial model of development through which we create our cities results in a double kill – i) decline in natural green spaces (which are pollution controlling agents that also provide health benefits) and, ii) increase in highly toxic pollutants. Thus, there is an increased influx of particular toxic chemicals such as oxides of nitrogen and sulphur, phosphates, nitrates and heavy metals that are commonplace in almost all contemporary cities. As a result, the quality of life is being severely affected. In 2019, the capital of India, Delhi, ranked 118 out of a total of 140 cities that were surveyed for the Global Liveability Index 2019. And this low rank was attributed to primarily deteriorating air quality. In relation to this, a recent study published in Nature Communications found black carbon particles on the fetal side of the placenta, an indication of the alarming consequences of increasing pollution levels on human health that is not limited to just respiratory disorders (Carrington, 2019).

In a previous blog post, we did touch upon the subject of nature and its influence on the mental health of the urban population. This led us to further explore the literature on the relationship between green spaces and human health. We found that there has been an overwhelming amount of work on this subject. The studies range from studying the effects of forest fragmentation on the spread of zoonotic diseases to finding linkages between exposure to nature and mental health, neurological outcomes as well as chronic illness such as cancer.

A meta-analysis of over 103 observational studies and 40 interventional studies found a significant relationship between exposure to green spaces and health benefits such as reduced incidences of Type II diabetes and stroke, cholesterol, blood pressure and salivary cortisol (a biological marker for stress). There are several hypotheses pointed out in this review article that can help explain this relationship. Firstly, access to green spaces provides opportunities for increased physical activity. Secondly, it provides opportunities for social interaction and exposure to sunlight that is a source of vitamin D, as well as serving as a buffer for air and noise pollution. There is another interesting hypothesis which is popularly known as “Old Friends”, which links microorganism biodiversity (including bacteria, protozoa and helminths) found in nature to enhanced immunity in human beings. Although human beings have evolved with these “good bacteria”, most of them are now either completely lost or our interactions with them is minimal due to an overall decline in plant and animal diversity in the urban world.

However, one of the most interesting mechanisms reported in this article, by which humans benefit from visiting natural green spaces is the production of “phytochemicals” (Twohig-Bennett & Jones, 2018). Rich biodiverse ecosystems produce a large spectrum of phytochemicals (chemicals produced by plants) that provide huge health benefits to all the living organisms in an ecosystem including human beings. Plant volatile compounds (secondary metabolites) help in building the human immune system. Forest atmosphere is majorly composed of compounds of terpenes such as pinene, limonene, linalool, etc. that are known to have positive physiological effects, including boosting the immune system, reducing stress, anxiety and depression, and fighting against chronic illness like cancer (by producing Natural Killer or NK cells).

Several studies reported that regular visits to natural green spaces is beneficial to human health. In countries like Japan, there is a culture of “forest bathing”. In 1982, the Forest Agency of Japan first proposed to include forest bathing as a part of people’s lifestyle. Quin Li (2010) conducted an empirical study to understand if forest bathing has any significant impact on the human immune system. A surprising find of this study was that forest bathing resulted in a significant increase in NK cells (reported to kill tumour or virus-infected cells), the effects of which lasted for almost 30 days after a visit to the forest. Did any of us ever imagine that trees could actually emit a mixture of chemical compounds that help build our immune system?

It surely seems that the diversity of forest ecosystems has a dire effect on human health. We could not help but wonder if invaded green spaces or restored biodiversity parks in Delhi also produce similar compounds that help human beings build their immunity. Maybe it is time we decipher the secret chemical language of plants.

References

1. Carrington, D. (2019, Sep 17). Air pollution particles found in the foetal side of placentas – study. The Guardian. Retrieved from  https://www.theguardian.com/environment/2019/sep/17/
2. Twohig-Bennett, C. & Jones, A. (2018). The health benefits of the great outdoors: A systematic review and metaanalysis of greenspace exposure and health outcomes. Environmental Research, 628-637.
3. Li, Q. (2010). Effect of forest bathing trips on human immune function. Environ Health Prev Med, 9–17.