7. Environmental Justice and Filtration Technology in Covering Highways
Health Impacts of Auto Emissions Near Highways:
The ultra-fine particulates and chemicals injected into the air from busy highways inflict premature deaths, chronic illnesses, birth defects, and other impacts on nearby residents and employees. 53,000 U.S. deaths annually are attributable to automobile emission air pollution. Many more people become chronically ill or incapacitated as a result of auto emission exposure and related illnesses. Ninety percent of the cancer risk from air pollution in Southern California is attributable to auto emissions.
Moreover, these health impacts are among the most unfair of preventable hazards to human health. The impacts are largely imposed on populations without their consent or knowledge. Children are disproportionately impacted. Biologically, children are more sensitive to pollutants and they present a longer life span for the health impacts to manifest. The Environmental Protection Agency (EPA) has recognized the need to site schools away from high emission roadways and install filtration in schools already located near such roads. The continued construction of multi-unit residential buildings in Los Angeles has been controversial. Of course, high emission urban roadways are lined with residences, businesses, and schools on a massive scale.
The health hazards of spending significant time near or in highway traffic are still not well understood by the public. If theses health hazards received attention proportional to other health hazards, it likely would lead to significant economic and social disruption. The habitability and value of homes and businesses within 1,000 to 2,500 feet of either side of highways would come under scrutiny. Near-highway zones could carry health warning signs, sales could require disclosures, and uses involving children could be banned or condemned. Instead, homes, businesses, and schools continue to be built near busy highways. Moreover, under the banner of transit oriented development (TOD) and city density goals, the residential developments near highways tend to be taller and denser than residences further away. Additionally, busy highways have historically been built in less economically influential neighborhoods and affordable housing tends to be built closer to the highways – resulting in an obvious health and safety bias against economically disadvantaged communities.
Ironically, most regulatory emphasis to date on filtration of cap and tunnel structures has been for the benefit of the automobile occupants under the cap or in the tunnel rather than for the benefit of the adjacent residents, school children, and employees. This seems unfair given that the adjacent residents, children, and employees have longer and more permanent exposure to the emissions, and perhaps less choice in the exposure.
Hopefully, the transition to electric vehicles and emission reduction requirements will significantly lessen this hazard by the time new highway airspace decks are completed. Nevertheless, air quality issues will persist. Legacy vehicles will continue to emit toxic particles and gasses for some time into the future. Additionally, the health impact of particulates from brakes and tires have not been fully studied.
Highway airspace deck structures, by virtue of their proximity to traffic, naturally raise issues concerning air quality. They also present opportunities to mitigate highway air pollution. Highway caps structures tend to reduce pollution adjacent to their midsections. This circumstance is accomplished by essentially trapping or redirecting the escape of particulates and gasses. Without more, this does not eliminate the pollutants, which may be concentrated in the below-deck tunnel and at the ends thereof. Decks also present opportunities to vent and filter pollution. European countries and Japan, with their greater number of longer tunnels, are leading the way in the research and development of effective treatment equipment.
Types of Filtration:
There are several types of filtration systems, some with different objectives. Most broadly speaking, there are systems designed primarily to remove particulates and there are systems designed to remove gases. Particulates have the greatest documented health impacts, and thus have been the focus of most filtration systems. These types of filtration systems can be further divided into two main types:
- Extraction systems that remove air from the tunnel or underpass, filtering particulates or gases on the way out. This type of system can utilize different types of filtration technology, including electrostatic filtration as well as biofiltration or bag filters. However, extraction systems may create an issue as to where the air is exhausted. Nevertheless, these systems tend to be used more often when the primary concern is pollution impact on the nearby community.
- Bypass systems that recirculate polluted air, after filtration, back into the tunnel. Most of the longest tunnels contain this type of system. It requires lengthy venting, either to the side of the roadway or above the road / under the deck. These systems tend to be used more often used when visibility and air quality within the tunnel are the primary concerns.
Both of these systems have been primarily designed for subterranean tunnels rather than tunnels resulting from man-made caps. However, the technology and impacts would seem to apply equally to highway caps.
Costs of Filtration Systems:
Cost data is uncertain and limited due to the relatively small number of existing filtration systems, their different technologies, different physical settings, different international locations, and different operating cycles. Operating costs tend to be high relative to installation cost. Energy consumption is generally the most expensive cost.
Capital costs in the period between 2000 and 2010 were in the range of $2 mil. to $5 mil. Annual operating costs for 24 hour operation, including energy consumption, were in the $500,000 range. However, most filtration systems were in operation far less than 24 hours per day – perhaps in large part due to the cost – but certainly filtration need changes with traffic volume, which changes throughout the 24 hour cycle. Cost may be significantly lower when installation of a filtration system is in the original design for cap or tunnel, rather than by retrofit at a later date.
Survey of Filtration Technology:
- Electrostatic Filtration: By far, this type of filtration is the most common in large tunnel applications. It uses electricity to charge (ionize) particles, which are then collected by electrodes – usually electrified plates – with an opposite charge.
- Bag Filters: Commonly used in industrial settings, bag filtration has a proven track record in those settings. However, they have proven less effective than electrostatic filtration in tunnel settings.
- Biofiltration: (see illustration below) In its infancy, current implementation is primarily to deodorize air in wastewater treatment. Researchers are investigating its application to emission pollutants – both particulate and gaseous. Such filtration involves a substrate including, e.g., sand or peat, select plants, and a “vegetated filter” and collection system. The benefits of such a system include low environmental profile – passive and renewable. Several French agencies carried out an experiment using biofiltration in a tunnel in France between 2012 and 2014. As stated in the CETU paper:
The biofilter was installed in the Guy Môquet tunnel, on the A86 motorway in Thiais (Val-de-Marne) – a 650-metre-long, bi-tube “cut and cover” tunnel with three traffic lanes in each direction, carrying 134,000 vehicles per day.
The experiment produced particulate filtration rates of between 30% and 90%, depending on particulate size, and 58% for NO2, with no loss of efficacy after 18 months. Biofiltration holds promise both as the sole filtration method or in combination with electrostatic filtration.
The Capitol Crossing highway cap project in the nation’s capitol, currently under construction, will feature “eco-chimneys that employ biofilters to clean exhaust and other toxins from the highway and integrated parking structure, returning clean air to the environment,” claims the architectural literature.
- Cold Plasma (in research and development, no current tunnel application) – This filtration technology involves using ionized gas to filter contaminated air. It holds promise for cost effectively filtering both particles and gases. It is likely to be some time yet before this technology is ready for tunnel applications.
Capitol Crossing “Eco-Chimneys.” Image courtesy of Property Group Partners.
Gas filtration systems are in much more limited use currently than particulate filtration systems. There are only four tunnels worldwide as of 2016 with gas filtration. These systems all focus on the treatment of NO2, a key ingredient in the creation of smog. In their implementation, these systems are typically integrated with a particulate filtration system – particulates necessarily being removed before gases. Because of the limited implementation of these systems in large scale tunnel applications, there is little data on effectiveness. The types of systems in use or in development include: 
- Absorption Denitrification: In this technology, treated gas undergoes a chemical reaction so that it will adhere to a solid surface. Activated carbon filtration is the most common form of this technology. In Japan, a pioneering filtration system of this type is using potassium hydroxide in the ChuoKanjo-Shinjuku tunnel near Tokyo. The system performance drops off by about 10% over 8 – 10 months but is easily regenerated.
- Adsorption Denitrification: In contrast to absorption denitrification, in adsorption denitrification (ab vs. ad) technology, gases are not altered or degraded before adhering to a solid material. Typically, this technology runs the gaseous air through containers holding pellets which “adsorb” the gases in their pores.
- Photocatalytic Denitrification: This technology uses titanium oxide (TiO2) and ultraviolet light to break down nitrogen oxides. The technology has proven fairly effective (with 15% – 30% NO2 reductions) in a tunnel in a Rome. However, there is concern about the health and environment impact of nanoparticles released in the process.
- Biofiltration of gasses is essentially the same system described earlier in this chapter for particulate biofiltration and has the same limitations.
- Cold Plasma to filter gasses (and also particulates) is promising for effectiveness, ease of implementation, and cost. However, it is still being researched. It is unlikely that this technology will be seen in a tunnel setting in the near future. In this process, an ionized gas (“plasma”) is used at ambient temperature (“cold”) to react with toxic gasses in such a way as to convert them into harmless gasses.
Maintenance, Cleaning, and Disposal:
All of these systems require periodic “regeneration” of the filter mediums. In the case of activated carbon, regeneration requires replacement after a period of 1 – 3 years. Several of the other systems are regenerated via automated cleaning processes. In these automated or ‘self-cleaning’ filtration systems, cleaning intervals generally occur every several days, with replacement required after several years. The cleansing solution is most typically water, which itself must be filtered before disposal. Where the solution is a chemical, additional measures are necessary.
As noted by the Tunnels Study Center (CETU) in France in its 2010 paper (updated 2017), The Treatment of Air in Road Tunnels – State-of-the-Art of Studies and Works:
“Before turning to systems that may effectively provide an answer to local pollution concerns, conventional ventilation techniques should still be considered by making use of the appropriate means, i.e. playing on the air flows and concentrations of the discharged vitiated air, as well as on the location and configuration of discharges and any other method likely to improve the dispersion of pollution and so protect the most at-risk areas.”
Some of the companies on the forefront of tunnel filtration are:
- Aigner (Austrian)
- Filtrontec (German)
- Panasonic (Japanese)
- CTA (Norwegian)
In conclusion, highway caps do not offer a global, or even a widespread, opportunity to mitigate highway emissions. They also are not a cost-effective method if mitigation is the only reason for building one. However, they do present an opportunity for mitigation in the limited urban locations where they are feasible. Additionally, as one of multiple reasons for building a cap, mitigation can help to justify construction of a cap. Moreover, emission mitigation impact can be valuable in gaining support for the construction of a highway cap.
Capitol Crossing sustainability features. Illustration courtesy of Property Group Partners.
 Adams, William “What is a safe distance to live or work near high auto emission roads?” UrbDeZine (2015) Retrieved from https://sandiego.urbdezine.com/2015/05/28/what-is-a-safe-distance-to-live-or-work-near-high-auto-emission-roads/
 Calazzo, F., Ashok, A., Waitz, I.A., Yim, S.H.L., Barrett, S.R.H. (2013) Air pollution and early deaths in the United States. Part I: Quantifying the impact of major sectors in 2005, Laboratory for Aviation and the Environment, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, August 2013. Retrieved on Jan. 22, 2018 from http://lae.mit.edu/wordpress2/wp-content/uploads/2013/08/US-air-pollution-paper.pdf
 Hulsey, B., Hopkins, E., Olson, E., Burg, E., and Carlson, M. (2004) Highway Health Hazards: How highways and roads cause health problems in our communities—and what you can do about it, Sierra Club, 2004, 6, par. 10. Retrieved on Jan. 22, 2018 from http://vault.sierraclub.org/sprawl/report04_highwayhealth/report.pdf
 U.S. Environmental Protection Agency (Nov. 2015), Best Practices for Reducing Near-Road Pollution Exposure at Schools, Retrieved on January 22, 2018 from https://www.epa.gov/sites/production/files/2015-10/documents/ochp_2015_near_road_pollution_booklet_v16_508.pdf
 Barboza, Tony and David Zahniser, L.A. officials push for new steps to address health risks from homebuilding near freeways (March 24, 2017) Los Angeles Times, Retrieved from: http://www.latimes.com/local/lanow/la-me-ln-freeway-pollution-study-20170324-story.html
 Best Practices for Reducing Near-Road Pollution Exposure at Schools, Supra
 Lau, Clement AICP (2012), Urban Freeway Cap Parks Policy Briefing Paper, (for Los Angeles Sustainability Collaborative), p. 19, retrieved from http://lasustainability.org
 Id. at p.15.
 See id. at p.20 re M5 East tunnel near Sydney.
 CETU, supra, pp. 28 – 29.
 CETU, supra, pp. 22 – 24
 Id. at pp. 26 – 27
 Id. at p. 24
 Id. at p. 30
 Id. at p.17
- New South Wales Government (Australia) Advisory Committee on Tunnel Air Quality – Roads and Maritime Services (July 2014) TP08: Options for treating road tunnel emissions, Retrieved on Jan. 22, 2018 from http://www.chiefscientist.nsw.gov.au/__data/assets/pdf_file/0008/54791/Road-Tunnels_TP08_Options_for_treating_road_tunnel_emissions.pdf
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