Green Roofs: Saving the Air and Saving Lives

Nicholas Lanni: Animal Science

Cole Payne: Building and Construction Technologies

Ben Lasky: Geology

Buzzing from the alarm clock’s warning ushers in the start of a new day. Avoiding the seductive snooze button, you roll out of bed and begin your morning preparations. Between the alertness of being awake and the daze of sleep, you slowly waddle to the dark bathroom to brush your teeth and wash off the last bit the previous night’s trance in the shower. Drying off, you begin to suit up for today’s task, whether its another day at school or a demanding shift on the job. Sizzling fragrant coffee provides the final jolt needed to get you on your way. Nearly walking out the door you almost forget something: your face mask. Leaving the safety of your climate-controlled home and venturing outdoors without it would be foolish. Indeed, taking deep breaths of unfiltered air is very unhealthy and dangerous.

Of course, One might be inclined to think that this hypothetical situation highlights an issue that does not concern anyone living in the United States. Air Pollution, might be a real hazard in impoverished countries thousands of miles away but how could it have any effects on those living in America? However, such a sentiment is, in fact, narrowminded. Globally, 6.5 million premature deaths were linked to air pollution in 2015 (Sifferlin, 2017). While deaths due to air pollution in the United States are relatively low compared to this number, research at the Massachusetts Institute of Technology revealed data that might come as a surprise. Emissions from industrial smokestacks, vehicle tailpipes, trains, and residential and commercial heating have contributed to fatal levels of air pollution. As of 2013, an average of about 200,000 early deaths were being reported each year due to prolonged exposure to pollutants in the air. Moreover, road transportation emissions were found to be the biggest contributor to premature deaths, followed closely by power generation. Road emissions caused approximately 53,000 early deaths, while power plant emissions lead to 52,000. These elevated levels of pollution were focused mostly along the east coast of the United States in urbanized areas and they disproportionately affect young children and the elderly (Chu, 2013). Furthermore, just seven years earlier, the American Lung Association estimated a much smaller annual figure for premature deaths due to exposure to elevated air pollution: 3700 (Yang et. al., 2008). Clearly these alarming estimates point to an ongoing and expanding problem in the United States.

Although you might consider avoiding this problem altogether by staying away from cities, you would be ignoring observed trends in economics and urbanization. In the year 2000 47% of the global population lived in Urban areas, compared to only 29% in 1950, and that figure is projected to exceed 61% by 2030 (Torrey 2004). Supplementing these figures, the United Nations Population Fund predicted that global urban population would rise from 3.3 billion in 2008 to 5 billion by 2030 (Yang et. al., 2008). However, this may actually be an underestimation, as more recent figures expect urban population to encompass as much as 80% of the world’s populace (Karteris et. al., 2016). Clearly, as economies grow and develop and more people migrate to urban centers in search of jobs, healthcare, and entertainment, cities will continue to grow (Boyd, 2017). More people in cities will lead to the potential for greater exposure to health risks associated with poor air quality. Jonathan Levy, professor of environmental health at Boston University, may best summarize the problem we face:

A public-health burden of this magnitude clearly requires significant policy attention, especially since technologies are readily available to address a significant fraction of these emissions. We have certainly invested significant societal resources to address far smaller impacts on public health. (Chu, 2013)

Resolving the issue of air pollution and its effects on public health will thus require sophisticated urban planning initiatives.

Prior to developing a proper solution for harmful air quality, it is necessary to understand the substances that are polluting the air and making it so dangerous for humans to breath. Commuters traveling into and throughout cities cause elevated traffic emissions which contribute to high levels of air pollution. The primary pollutants dispersed by road traffic are nitrogen oxides, mainly nitrogen dioxide, and particulate matter. Particulate matter (PM10) typically describes a mixture of solid particles and liquid droplets that are small enough to be suspended in air. This can include dust, smoke, pollen and mold. PM10 is a category of inhalable particles that are so small (diameter of 10 micrometers), it would take at least 10 individual particles to encircle a single strand of human hair. These particles are the main cause of low visibility, or haze, in the United States. They are also small enough to enter human lungs and lead to health problems such as asthma and premature death for people with lung or heart disease (Environmental Protection Agency, 2016).  Sulfur dioxide (SO2), together with nitrogen dioxide (NO2), are toxic gases spewed out of power plants as well as automobiles. Finally, ozone (O3) is another dangerous pollutant that forms from chemical reactions with sunlight and air pollutants that come from cars, power plants and chemical plants. Ozone causes muscles in the airways to squeeze and trap in air leading to shortness of breath. Such effects can compound lung diseases like asthma and are more common in children and the elderly (Environmental Protection Agency, 2016). Each of these pollutants are tied tightly to transportation because, in large part, they come directly from the exhausts of cars. Since cities are economic hubs, people traveling there for work each day contribute to elevated levels of air pollution and an overall degradation of air quality.

Furthermore, there are six primary emission sectors that have been identified by the Massachusetts Institute of Technology. These include electric power generation, industry, commercial and residential sources, road transportation, marine transportation, and rail transportation. Its worth repeating that the greatest cause of air pollution-related premature deaths in the U.S., 53,000 per year, is from road transportation emissions. These come from the exhaust seeping through the tailpipes of cars and trucks (Chu 2013). Unfortunately, pollution derived from cars and trucks is rising in Massachusetts. The Department of Environmental Protection updated its inventory on emissions, and they found a spike in such contamination in Massachusetts due to transportation. There has been an increase in emissions measured at 6% between 2015 and 2016. In fact, pollution due to transportation is higher in this state than at any point over the decade. Such a trend is due to the over 400,000 more jobs in Massachusetts that did not exist just ten years ago. As jobs are on the rise, there are also more commuters, higher levels of gasoline consumption, and more air pollution (Gatti, 2018). On the other hand, public transportation in Massachusetts, which would ideally reduce emissions by requiring fewer people to use their own cars or trucks, is down 6.5% for buses and 3.5% for trains. This could potentially be due to the rise in competition due to ridesharing services that include Uber and Lyft (Gatti, 2018). Urbanization, which we define as the massive migration of people to cities and the subsequent increase in urban populations, is a part of a global trend that is easily observable in the growing population of Boston. The city’s thriving economy is regrettably creating some undesired consequences in the form of this dangerous rise in pollution.

Therefore, this paper focuses on the goal of addressing air pollution concerns. Green infrastructure, particularly green roofs, have demonstrated the potential for improving urban environments (Currie & Bass, 2008; Yang et. al., 2008). In order to manage and control air pollution, reduce loss of life due to health hazards arising from emissions, foster increased awareness of green roof potential and provide data for green roof performance, we propose that Boston adopts a mandate that all municipal and public buildings be required to install green roofs. A mandate involves a policy action by the Boston municipal government that authorizes and commands all publicly owned buildings to be outfitted with green roof systems. The aim of such a directive is to address current air quality issues and promote the development of the green roof industry.

Before understanding the effectiveness of green roofs in managing and reducing urban air pollution levels, it is critical to understand what comprises a green roof. A green roof is a system of multiple layers.  It involves covering a roof in a layer of vegetation that grows out of a lower layer composed of growing material such as soil. This material provides the source of nutrition for the vegetation above and can vary depending on the local climate and plant selection. Below the growing material there is a layer of loose inorganic material such as shredded rubber which works to retain water. A waterproof layer sits beneath this to prevent leakage to the interior of the building (Wang, Tian, & Zhao 2017). There are two types of green roofs, extensive and intensive. Intensive green roofs are deeper than 20cm and can accommodate larger plants such as small trees and shrubs but are more costly to install and maintain since they require irrigation and additional structural support (Williams et. al., 2010). Extensive green roofs are shallower and are limited in plant selection; using grasses, moss, and succulents yet are more widely implemented due to their lower cost of maintenance associated with little to no irrigation and their overall lighter weight load (Shafique et. al. 2018).

While Boston does already have a green roof presence on some buildings, including Boston City Hall, Massachusetts General Hospital, and the east office building at the World Trade Center in Boston (Green roofs and stormwater management, 2018), this type of green infrastructure has not yet been utilized or studied on the same scale as in other cities. Toronto, for example, has one of the more expansive green infrastructure strategies. Estimates illustrate that 109 hectares of green roofs in this city may remove a total of 7.87 tons of air pollutants annually (Currie & Bass, 2005). That measure of microscopic particles and seemingly weightless toxic gases such as Nitrogen oxide is nearly the combined weight of 4 African Elephants (National Geographic, 2018). Estimates in Washington D.C. were even more optimistic: if all the capital’s roofs were converted to green roofs then about 52.6 metric tons of air pollutants could be removed (Corrie et. al. 2005). Furthermore, the same study that produced these estimates found that covering 20% of the roof surface in Chicago would reduce the Nitrogen Oxide gases in the air by a range of 806 to as high as 2769 metric tons. Imagine that stampede of elephants. These sources agree, air quality and therefore public health can be improved by green roofs by drawing down levels of toxic gases.

Chicago serves as another exceptional example of effective green roof initiatives. In a study from 2007 The American Lung Association stated that 2 million of the roughly 2.9 million people living in Chicago were at a heightened risk for health issues that were the direct result of exposure to ozone and particulate matter. As of that year, Chicago was the number one ranked North American city in terms of combined area of green roofs (Yang et. al. 2008). Ozone was the main air pollutant in Chicago with particulate matter ranked second. Particulate matter and ozone concentrations peak in the summer whereas sulfur dioxide and nitrogen dioxide levels peak in the winter (Yang et. al. 2008). Research discovered that 19.8 hectares of green roofs, nearly 20 soccer fields, removed about 52% of ozone from passing air, 27 % of nitrogen dioxide, 14% of particulate matter and 7% of sulfur dioxide (Yang et. al. 2008). Such a drop-in pollutant levels in this leader in green roof implementation is a welcome benefit for those suffering or at risk of suffering from respiration issues that are made worse by these contaminants.

Additionally, green roofs can effectively address the issue of particulate matter. The tiny particles carried by the wind stick to the leaves and plant stems of the green roofs. Moreover, nitrogen dioxide and sulfur dioxide that are carried on air currents can be trapped and dissolved through the stomata of plant leaves. Stomata are the pores, or holes, on the skin of leaves that allow a plant to absorb gases (Currie and Bass 2008). In fact, just one square meter of green roof coverage can offset the annual particulate matter emission of one car (Claus & Rousseau, 2012).

Installation and maintenance costs are the primary barriers to implementation of green roofs. However, high initial costs can be justified through the long-term benefits produced by green roof systems. Construction prices for green roofs can range from $30-$75 more per square meter than conventional roofs (Green roofs and stormwater management, 2018).  In addition to all the work they do to clean the air, green roofs also have additional economic benefits that help offset their costs over the long run. These include reducing stormwater runoff treatment (Bliss et. al., 2009) and energy savings (Getter et. al., 2011; Teotonio et. al., 2018; Claus & Rousseau, 2012; and Yang et. al., 2008). One example of energy savings is highlighted in research conducted In Michigan. An experimental study in several towns in Michigan that exhibit a climate that experiences hot humid summers and cold, snow-filled winters displayed that green roofs reduced heat escaping the building by an average of 13% in the winter and 167% during summer (Getter et. al. 2011). This is especially important due to the similarities of this climate with the climate of Boston. A separate study in Portland, Oregon showed reductions of heat loss by about 13% in the winter and around 72% in the summer (Spolek 2008). We can apply these reductions in escaped heat easily to energy savings. When heat is conserved inside of buildings less energy is required to maintain comfortable indoor climates. The Massachusetts department of Energy Resources tracks average household heating costs depending on fuel type used. Natural gas costs $983 per year, Heating oil costs $2,359 per year and propane comes out to $1,808 each year (Household heating costs, 2018). Using the 13% reduction in energy consumption due to heat provided by green roofs, that’s an annual cost savings of about $128 for natural gas, $307 for oil, and $235 for propane.

Besides energy savings, green roof monetary benefits are highlighted in at least one more direct and easily measurable way. Green Roof service lives are usually 40 years whereas traditional roofs are replaced, on average, after 20 years (Teotonio et. al. 2018; Claus & Rousseau, 2012). By effectively doubling the lifespan of a roof, a city or building owner can avoid costly repairs for longer intervals of time.

Perhaps equally important is green roofs positive impact on stormwater management. While this is not directly related to air quality, green roof effects on rainwater runoff display potential to save money for the city in which it is located. First, green roofs do not require additional space to be implemented into a water sensitive urban design measure. They are already a part of the building’s existing footprint. Conventional roof surfaces account for an average of nearly 50% of impervious surfaces in urbanized areas (Karteris et. al. 2016). These surfaces prevent water from being absorbed into the ground and can therefore be hazardous as they lead to the buildup of stormwater runoff (Shafique, Kim & Rafiq, 2018). Such a buildup can intensify flood events in addition to taxing stormwater infrastructure in the city. Green roofs, being a part of smart urban design by controlling and managing the sheer volume of rainwater, can thus prevent damage due to flooding on a city-wide scale. Green roofs displayed water retention values between 60-90% (Wang et. al. 2017). Additional research displayed similar results as runoff was reduced by 45-71% (Bliss, Newfield, & Ries 2009). Retention capacities of green roofs, which hold onto rain like a sponge, allow greater control of urban runoff and reduce the stress on conventional stormwater infrastructure while preventing possible flood events.

Of course, in order to gain access to any benefit afforded by green roofs, there must be active participation by both public and private interests. Policies aimed at stimulating green roof development have been partially successful in cities like Chicago and Portland. In fact, the success of green roof policy in Portland, Oregon is a primary example. In the years between 2001 and 2006 there were 26 green roofs constructed, which were built to meet stormwater management goals (Carter and Fowler 2008). These projects were the result of publicly funded incentives that were meant to encourage private investment in green roof development. The authors of this paper, however, believe that the quickest way to jumpstart the implementation of green roofs is by converting public buildings to green roof systems through a city-wide mandate. This allows for the policy makers to back a green initiative and address the public health issue that is air pollution. City owned buildings, such as schools, government offices, and police and fire stations, would provide a sizeable investment and help address the lack of research that would otherwise provide confidence in the economic and environmental benefits of green roofs (Williams et. al., 2010). Mandating that public and municipal buildings be covered with green roofs also avoids the politically unpopular approach of enforcing guidelines on privately owned buildings. Boston’s nearly 200 city-owned buildings (Green roofs and stormwater management, 2018) could establish local green roof examples to serve as references for future public or private installations. Moreover, increasing the scale of green roof impacts by converting public buildings would increase the awareness of the technology, provide a source for data on local and site-specific performance of green roofs, particularly in air pollution control, and set the city on course for improving the health of the population by managing and drawing down air pollution levels.

Growing populations in cities stand to suffer from increases in harmful emissions and air pollution. Green roofs have been discussed as an effective measure for mitigating the negative impacts that result from otherwise unbridled urbanization and they also have the potential to reverse negative effects on air quality. The merits of green roofs, or rather the effectiveness of green roof systems, must be established prior to gaining the support of the voting public. This would involve illustrating the usefulness of green roof systems in addressing the issues of air pollution. The city’s population would then be convinced of the benefits and effectiveness of green roof systems prior to placing their support behind any large-scale policy aimed at directing public funding into green infrastructure. Research into the capabilities and potential flaws involved with green roofs has presented plenty of evidence of the positive impacts and challenges they create. The studies observed above made progress in examining green roof effects on stormwater management, energy consumption through heating and cooling buildings, and mitigation of air pollution. However, these studies focused on individual cities in only a few areas throughout the globe and were executed on relatively small scales. Further study of broader applications of green roofs in diverse environments would be wise before fully committing to large scale implementation. Mandating the conversion of Boston’s public buildings to green roofs allows for a local baseline to be established which could provide data on the local effectiveness of green roofs, allow for the spread of green roof potential, and begin Boston on  path to larger scale green roof implementation while addressing the public’s interest in controlling and reducing air pollution.



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  2. I enjoyed your article. This is my first time really seeing and reading about green roofs, it was unknown phenomenon to me previously. Seems like a very smart way of improving air quality!

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