**7. Discussion**

The preceding analysis suggests that the more significant microclimatic factors are sound, carbon dioxide concentration and dust concentration. These factors were continually associated with high feature importance scores in both physical and

*The Life2 Well Project: Investigating the Relationship between Physiological Stress… DOI: http://dx.doi.org/10.5772/intechopen.107493*

#### **Figure 8.**

*Shapley summary plot of microclimatic factors and self-reported mental state.*

mental well-being measures, and in both objective scores recorded from electronic instruments and more subjective self-report forms.

We acknowledge that the true influence of each value is much more abstract than the assigned Shapley value due to the 'black box' nature of random forest regression; further we also acknowledge that model interpretability does not mean causality. Notwithstanding these caveats, we have chosen to share the Shapley plots in this paper because they allow us to examine the contributions of features in ways that can be easily understood and used to make inferences.

The associations reported in this paper bear out causal linkages reported by similar studies in the field. For example, Kessel et al. [21] have documented positive relationships between ambient temperature and both body core temperature as well as corneal temperature, and Che Muhamed et al. [22] have documented positive relationships between relative humidity and body core temperature.

As for heart rate, Randall and Shelton [23] found that "carbon dioxide excess causes an increase in ventilation volume by virtue of a greater depth of breathing, the frequency decreasing slightly. The heart rate goes up with increasing carbon dioxide concentrations". Verberkmoes et al. [24] have shared that "the influence of atmospheric pressure and temperature on the incidence of acute aortic dissections may be explained by an increase in sympathetic activity, which is responsible for higher blood pressure, and heart rate". Another factor that affects heart rate is dust concentration. As suggested in research by Pope et al. [25], elevated particulate levels were associated with increased mean heart rate, and decreased overall heart rate variability. Sound also plays a part in affecting heartbeat, according to past research – for example, [26] – the higher the noise level, the higher the heart rate. Heart rate drops at night when humans are sleeping. According to Ahmed [27], "a balance of impulse from the sympathetic and the parasympathetic nerves determine a person's baseline heart rate. Interestingly, in experiments where a person's nerve supply is blocked, the heart rate is often higher; this would suggest that the parasympathetic nerve impulses that serve to slow the heart rate down are the predominant force under normal resting conditions. This is particularly evident at night when most people have a significant drop in heart rate".

Heart rate variability has been shown to be influenced by carbon dioxide concentration [28]. Our findings are congruent with studies which have elaborated on how and why heart rate variability is decreased with higher dust concentration [29], and with higher noise levels [30].

In terms of the associative relationships between microclimate and mental health, Mullins and White [31] have observed that "we find that higher temperatures increase emergency department visits for mental illness, suicides, and self-reported days of poor mental health". In a similar paper by Sygna et al. [32], results suggested that "individuals with poor sleep quality may be more vulnerable to effects of road traffic noise on mental health than individuals with better sleep quality". This position is congruent with that of the present study, in which extremes of sound levels were associated with both lower mental well-being and lower sleep quality (the latter as measured through sleep score).

As the present paper is inspired as a response to anthropogenic climate change, the extent to which carbon dioxide concentrations affect health and well-being is of interest. Kajtár et al. [33] have reported that well-being - as well as capacity to focus attention - both decline when carbon dioxide concentration in the air increases nearly tenfold to 3000 ppm.

The preceding parallels between earlier studies and the present study are encouraging, because a foundational paradigm driving our work was that of citizen science and the democratization of small-scale, low-cost research as enabled by data science and the Internet of Things (IoT). We assembled the wearables from off-the-shelf parts and coded them ourselves. Technology has enabled such democratization not only in terms of the (relatively) low cost of hardware and the open source movement in general, but also in terms of recent developments in data science and machine learning. The latter have meant that the large and burgeoning datasets associated with the use of IoT can be accessible and intelligible to wider cohorts of students and researchers. The parallels be-tween our work and earlier studies suggest that – going forward, in a world where anthropogenic climate change is a (sad) reality – meaningful scientific and geographic inquiry can be undertaken by a much wider crosssection of the general public than was once previously possible.

Several opportunities therefore suggest themselves for potential future work, which could take the form of either scaling up or translation to investigate other microclimatic variables (such as the role of infrared radiation on well-being), biometric indicators (such as readings from electroencephalograms) and socio-demographic *The Life2 Well Project: Investigating the Relationship between Physiological Stress… DOI: http://dx.doi.org/10.5772/intechopen.107493*

contexts. These and other possible avenues for future work will go some way to addressing the limitations of the present study, foremost among which was the movement restrictions associated with the necessity to follow COVID-19 protocols throughout the duration of the study. These movement restrictions meant that the microclimates sampled were necessarily limited in variety. It is hoped that possible future extensions of this study in 2022 may not be as constrained.
