Mum, I’m Going on a Bike Ride. Okay Darling, But Don’t Forget Your Mask.

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Figure 1: Is this what we’re likely to see in the future? 

ounds strange doesn’t it? It’s not the first thing you would expect to say before they leave the house, but it could become a reality. We all know that exercise is good for you, as Hippocrates once said, “Walking is [a] man’s best medicine”. But is that still valid, can the negative effects of air pollution outweigh the benefits of exercise? The answer to this will be explored during this article, but firstly lets marvel at the spectacle of exercise. It’s quite unique if you think of it: you feel great afterwards, guilty for avoiding it, but hate every minute while you’re doing it. I suppose it’s the perfect antidote to chocolate, as the reverse behaviour can be observed.

The benefits of exercise are well known, it has shown to protect against memory loss, cancer, retinal degeneration, type 2 diabetes and cardiovascular disease, inhibit the initiation and growth of tumours, while improving brain function and cognition [1, 2]. There are also psychological benefits: including lowering an individual’s perception of stress and helping that individual manage their anxiety and/or depression [3, 4]. However, it is important to highlight that individuals with higher self-efficacy, who exhibit fewer depressive symptoms have a greater perceived value of the outcomes of exercise [5]. This means that these individuals are more likely to partake in exercise compared to those who suffer from depression or low self-esteem. De Mello et al. (2013) also came to this conclusion from a survey of adults in Sao Paulo, Brazil.

In all, I think it’s safe to conclude that exercise is an important ingredient of a healthy lifestyle and we should all probably do a little bit more. But that doesn’t answer the question: does air pollution counteract the health benefits of exercise? To answer this, we firstly need to understand what is air pollution and how does it affect our health. For that I recommend you quickly read my previous article titled “Air Pollution And Health: Are Your Allergies Getting Worse?” Air pollution can come from various sources because of natural processes or human activity; a main source of air pollution comes from burning fuels: either for electricity, transport or heat.

To analyse whether air pollution can counteract the benefits of exercise we need to determine if there exists a breakeven point: by that I mean, how long you would have to exercise in-order to accumulate enough toxins to have an overall negative impact on your health; whereas the tipping point, is defined as the point at which any additional exercise will reduce the overall benefit of conducting that exercise. The tipping point can also be viewed as the optimum length of time to exercise at a particular location under the current conditions. The tipping point and the breakeven point are demonstrated on the graph below. Positive health effects correspond to a reduction in the relative risk of death, while the negative health effects correspond with an increased relative risk of mortality. These plots also assume that the individual participates in these activities consistently on a daily or weekly basis, depending on the choice of scale.

Figure 2: Demonstration of the breakeven and tipping points. This is not based on any real-world data, it is rather a representation of the general trend.  

This can be achieved by firstly converting each mode of exercise into its Metabolic Equivalent Task (MET), a method of estimating the amount of oxygen required during physical activity. This value varies depending on the activities’ intensity and one’s physique.  For example, walking, generic cycling and the quality time spent with your partner have MET values of 3, 7.5 and 1.8 respectively [7]. The reduced risk of death from participating in these activities for a chosen duration of time, can be determined from their corresponding dose-response curve. With regards to the dose response curves, for a particular duration of time a value less than one indicates a reduction in risk; whereas the opposite is true for a value greater than one.  A value equal to one is equivalent to the individual at rest, thereby not participating in any physical activity during that time period. For more information about dose-response curves see the paper authored by Kelly et al. (2014).

The level of air pollution varies significantly depending on many factors, including traffic density and composition, referring to the number and types of vehicle on the road; meteorological conditions, referring to the weather [9]; time of day, this is related to traffic; time of year, as light intensity affects the rate of photochemical reactions; and tree density, as trees can reduce the concentration of coarse particulates [10].

The World Health Organisation has measured and estimated the background concentration of PM_2.5 for over 1600 cities around the world, and this data can be found in the Ambient Air Pollution database [11]. Interestingly enough, using the data available from the database Glasgow, not London, had the highest measured ambient concentration of PM_2.5, at 16 μg m^-3 (measured in 2013 by the European Environment Agency). Zabol in Iran, has the world’s highest recorded annual concentration of PM₂.₅ at around 217 μg m^-3. Zabol’s air pollution is heavily inflated by high temperatures and continuous dust storms throughout the summer months [12].

The World Health Organisation (2014b) have reported that for every 10 μg m^-3 increase of PM_2.5­ in ambient air, increases the relative risk of mortality by 1.07. Therefore, the benefits of exercise and the detriments due to air pollution can be quantified as relative risks of mortality. With increasing duration and intensity of the activity, the amount of oxygen required to complete the activity increases. The amount of inhaled air pollution also increases proportionally with increasing oxygen requirement, assuming uniform local air composition. For more information about the model required to determine the breakeven and tipping points for a given activity and fine-particulate concentration (PM₂.₅), see the paper authored by Tainio et al. (2016).

Tainio et al. (2016) concluded that for most of the world, the health benefits of exercise greatly outweigh the detrimental effects of air pollution experienced during that period. For example, if you were to go out on a 30-minute bike ride, the background PM₂.₅ concentration would need to be 95 μg m^-3 to reach the tipping point, or 160 μg m^-3 to reach the breakeven point. To reach the tipping point for the world’s average PM₂.₅ concentration, one would have to cycle for 7 hours or walk for 16 hours a day [14]. So, if you’re walking more than 16 hours a day, then you have a legitimate excuse to put your feet up, if the blisters don’t get you first. 

On the other end of the scale, New Delhi has one of the highest pollution levels in the world. With an annual average PM₂.₅ concentration of 153 μg m^-3, the tipping and breakeven points can be reached within 30 and 45 minutes of cycling per day, or 90 and 285 minutes of walking per day [15]. It’s important to highlight the fact that these are not peak concentrations, but yearly averages; therefore, to meet the UK’s recommendation of “at least 150 minutes of moderate aerobic activity such as cycling or fast walking” [16], in cities like New Delhi, gas masks maybe required.

Finally, there is one more important observation to make with regards to the dose-response curves. The dose-response curves were originally developed from a literature review of studies on the relative risks of all-cause mortality (in other words risk of death) of each activity. From the 21 studies that were included: 13 came from Western Europe, 4 from the USA, 3 from China and 1 from Japan [8]. In total, there were 467 000 participants with ages ranging from 20-93 and a mean of 56.6 years [8]; however, this does not imply unique participants as there might be overlap between the two sets of studies.

Choosing to include these studies presents two potential problems: firstly, these studies were carried out in predominantly western cities and secondly, the average age of the participants is significantly greater than the estimated world average of 32.4 years. This estimate was calculated from census data provided for each year group by the U.S. Census Bureau (2016). The population figures are based on the predicted world population (as of June 2017) that was originally made in 2014 and revised in 2016. These two problems may produce unrepresentative dose-response curves, which cannot be used for the entire world, as air pollution affects different age groups to varying degrees and there might be other regional health factors that were not considered during these studies.

Another criticism of the paper authored by Tainio et al. (2016), was that there was no attempt to quantify the effects of other pollutants that contribute to air pollution [15]. Air pollutants such as Carbon Monoxide, Ozone and Nitro Oxides have all been shown to have detrimental effects on our health; therefore, the reported exercise durations required to reach the tipping and breakeven points, are likely to be lower and even more complexly dependent on where you live compared to that suggested by only considering PM₂.₅.

So, is the solution to wear a face mask? Well, that might not solve the problem; besides being uncomfortable, there simply hasn’t been enough research carried out to conclusively state whether wearing a face mask in this application would be beneficial. Studies have shown that wearing an effective face mask in polluted cities, can lower blood pressure, improve heart variability and reduce ventilation rate [18-20]. Sounds great, right? So, what’s the problem? Well, these studies were focusing on low intensity activities and their findings might not be applicable for high intensity activities. There are concerns that face masks restrict ventilation, thereby reducing the oxygen supply to the body, which becomes increasingly important as the intensity increases [21]. There are also many more unanswered questions that needs to be explored before a decisive conclusion can be made.

To conclude, literature states that air pollution can negate the health benefits of exercise, but only in extreme conditions achieved by a few cities worldwide. There is also insufficient data to draw meaningful conclusions with regards to the benefits and drawbacks of wearing face masks, in the application of exercising in polluted cities. If you would like to get your own face mask, then check out these webpages [22, 23]. But I can’t help to think, it would be more decisive to focus on reduction and prevention strategies rather than mitigation. What would you rather focus on?

References

1.      Febbraio, M. A. (2017) Exercise metabolism in 2016: Health benefits of exercise – more than meets the eye! Nature Reviews Endocrinology. 13. p. 72-74. DOI: 10.1038/nrendo.2016.218.

2.      Pedersen, L. et al. (2016) Voluntary Running Suppresses Tumor Growth through Epinephrine- and IL-6-Dependent NK Cell Mobilization and Redistribution. Cell Metabolism. 23 (3). p. 554-562. DOI: 10.1016/j.cmet.2016.01.011.

3.      Sanches, A. et al. (2016) Relationship among stress, depression, cardiovascular and metabolic changes and physical exercise. Fisioterapia em Movimento. 29 (1) p. 23-26. DOI: 10.1590/0103-5150.029.001.AO02.

4.      Otsuka, T. et al. (2016) Effects of acute treadmill running at different intensities on activities of serotonin and corticotropin- releasing factors neurons, and anxiety- and depressive-like behaviours in rats. Behavioural Brain Research. 298. B. p.44-51. DOI: 10.1016/j.bbr.2015.10.055.

5.      Marszalek, J. (2017) Outcome Expectations and Osteoarthritis: Association of Perceived Benefits of Exercise With Self-Efficacy and Depression. Arthritis Care & Research. 69 (4). p. 491-498. DOI: 10.1002/acr.22969.

6.      De Mello, M. T. et al. (2013) Relationship between physical activity and depression and anxiety symptoms: A population study. Journal of Affective Disorders. 149 (1-3). p. 241-246. DOI: 10.1016/j.jad.2013.01.035.

7.      Ainsworth, B. E. et al. (2011) Compendium of Physical Activities: a second update of codes and MET values. Medicine and Science in Sports and Exercise. 43 (8). p. 1575-1581.

8.     Kelly, P. et al. (2014) Systematic review and meta-analysis of reduction in all-cause mortality from walking and cycling and shape of dose response relationship. International Journal of Behavioural Nutrition and Physical Activity. 11:132. DOI: 10.1186/s12966-014-0132-x.

9.      Pleijel, H. (2016) A method to assess the inter-annual weather-dependent variability in air pollution concentration and deposition based on weather typing. Atmospheric Environment. 126 (2). p. 200-210. DOI: 10.1016/j.atmosenv.2015.11.053.

10.   Selmi, W. et al. (2016) Air Pollution removal by trees in public green spaces in Strasbourg city, France. Urban Forestry & Urban Greening. 17 (1). p. 192-201. DOI: 10.1016/j.ufug.2016.04.010.

11.   World Health Organisation (2014a) Ambient Air Pollution Database. [Online] Available from: http://apps.who.int/gho/data/view.main.AMBIENTCITY2014?lang=en. [Accessed: 10/07/2017].

12.   The Guardian (2016) Which are the world’s two most polluted cities – and why? [Online] Available from: https://www.theguardian.com/environment/2016/may/12/which-are-the-worlds-two-most-polluted-cities-and-why. [Accessed: 10/07/2017].

13.   World Health Organisation (2014b) WHO Expert Meeting: Methods and tools for assessing the health risks of air pollution at local, national and international level. [Online] Available from: http://www.euro.who.int/__data/assets/pdf_file/0010/263629/WHO-Expert-Meeting-Methods-and-tools-for-assessing-the-health-risks-of-air-pollution-at-local,-national-and-international-level.pdf?ua=1. [Accessed: 10/07/2017].

14.   Tainio, M. et al. (2016) Can air pollution negate the health benefits of cycling and walking? Preventive Medicine. 87. p. 233-236. DOI: 10.1016/j.ypmed.2016.02.002.

15.   Woodward, A. & Samet, J. (2016) Active transport: Exercise trumps air pollution, almost always. Preventive Medicine. 87. p. 237-238. DOI: 10.1016/j.ypmed.2016.03.027.

16.   NHS (n.d.) Physical activity guidelines for adults. [Online] Available from: http://www.nhs.uk/Livewell/fitness/Pages/physical-activity-guidelines-for-adults.aspx. [Accessed: 10/07/2017].

17.   U.S. Census Bureau (2016) International Data Base World Population by Age and Sex. [Online] Available from:  https://www.census.gov/population/international/data/idb/worldpop.php.  [Accessed: 10/07/2017].

18.   Langrish, J. P. et al. (2009) Beneficial cardiovascular effects of reducing exposure to particulate air pollution with a simple facemask. Particle and Fibre Toxicology. 6:8 DOI: 10.1186/1743-8977-6-8.

19.   Langrish, J. P. et al. (2012) Reducing Personal Exposure to Particulate Air Pollution Improves Cardiovascular Health in Patients with Coronary Heart Disease. Environmental Health Perspectives. 120 (3). p. 367-372. DOI: 10.1289/ehp.1103898.

20.   Barrett, J. R. (2012) Air Pollution Intervention: Study Links Use of Face Masks to Improve Cardiovascular Outcomes. Environmental Health Perspectives. 120 (3). a122. DOI: 10.1289/ehp.120-a122b.

21.   Wang, Z. (2016) Air Pollution and Exercise: A Perspective From China. Research Quarterly for Exercise and Sport. 78 (3). p. 242-244. DOI: 10.1080/02701367.2016.1203270.

22.   Saint Cyr, R. (2014) Air Pollution Masks: A Review of the Best. [Online] Available from: http://www.myhealthbeijing.com/china-public-health/air-pollution-masks-a-review-of-the-best/. [Accessed: 11/07/2017].

23.   Everington, K. (2013) Where’d You Get That Mask? Update. [Online] Available from: http://www.theworldofchinese.com/2013/04/whered-you-get-that-mask/. [Accessed: 11/07/2017].