Bevin P. Engelward

Predicting health impact of environmental contaminants for environmental justice communities in Eastern Maine: The longer mercury-controlling policies are delayed, the less effective they will be. Furthermore, the current administration is proposing to roll back existing regulations on mercury emissions. Mercury (Hg) is emitted into the atmosphere by multiple natural and anthropogenic sources (e.g., coal-fired power plants). Mercury is of global concern owing to its persistence in the environment, its ability to be transported far away from emission sources, and its bioaccumulation to toxic levels in food webs. Mercury emissions are addressed under the global 2017 Minamata Convention, which requires that countries control emissions from specific sources. Policymakers typically use models to evaluate their emissions-reducing plans. However, most of these models do not appropriately account for legacy emissions (recycling of previously deposited Hg) and the fact that today’s anthropogenic emissions are tomorrow’s legacy emissions. Hélène Angot, under the supervision of Prof. Noelle Selin (Project 2), developed an integrated modeling approach to account for legacy emissions given various emissions-reducing policy timelines (Angot H, Hoffman N, Giang A, Thackray CP, Hendricks AN, Urban N, Selin NE. Global and local impacts of delayed mercury mitigation efforts, Environmental Science & Technology, https://doi.org/10.1021/acs.est.8b04542). While the Minamata Convention urges that countries take action as soon as possible, its requirements for controlling sources allow for up to a 10-year delay. The MIT SRP team found that for every five-year delay, the impact of policy measures will be reduced by 14% on average. They also found that delays will have serious consequences in regions that are far from emission sources, such as tribal areas of Eastern Maine, and they discussed implications relevant to environmental justice concerns. The approach outlined in this work provides a new and straightforward methodology to better inform policy decision-making, as well as providing a platform for ongoing studies of MIT SRP contaminants of interest, including polycyclic aromatic hydrocarbons.

Development of Carbon Nanotube Sensors for the Detection of NDMA: In 2018, Maggie He, under the supervision of MIT Superfund Prof. Tim Swager (Projects 1 and 2), published the development of carbon nanotube sensors for the detection of γ-radiation (Zeininger L, He M, Hobson S, Swager T. Resistive and capacitive γ-ray dosimeters based on triggered depolymerization in carbon nanotube composites. ACS Sensors 3, 976–983, 2018, PMID: 29558118) and amines (Paoletti C, He M, Salvo P, Melai B, Calisi N, Mannini M, Cortigiani B, Bellagambi FG, Swager TM, Francesco FD, Pucci A. Room temperature amine sensors enabled by sidewall functionalization of single-walled carbon nanotubes. RSC Advance 8, 5578–5585, 2018). These publications describe the functionalization of single-walled carbon nanotubes with responsive units, poly(olefin sulfone)s and tetrafluorobenzoic acid, which can impart sensors with high sensitivity to γ-radiation and amines, respectively. These carbon nanotube chemiresistive sensors function at room temperature and have the advantage that they require very low power, provide real-time detection, are inexpensive, and can be made portable for field deployment. The design principles and techniques developed for this work provide a foundation for which the MIT SRP N-nitrosodimethylamine (NDMA) sensors will be designed and fabricated. Distributing portable sensors at contamination sites will allow SRP to obtain spatiotemporal concentrations of NDMA. This information will help nearby communities to minimize exposure and will be useful to optimize efforts for site remediation.

Recruiting under-represented members of our community to join the Superfund Research Program: Prof. Bevin Engelward (MIT SRP Program Director) attended the 2018 Annual Biomedical Research Conference for Minority Students (ABRCMS) in order to recruit under-represented members of our community to become involved in research related to environmental health, and in particular to tell them about the NIEHS Superfund Research Program. Over 4,000 faculty and students attend the ABRCMS meeting. Most of the attendees are undergraduates who are considering different options for graduate studies. This makes this meeting particularly effective for reaching out to promising students at an early stage in their career to encourage them to consider a career focused on public health. Prof. Engelward reached out to dozens of students to tell them about the SRP, and in particular about the research at MIT. Many students were not aware of environmental health research, and thus benefited from learning about the ongoing work at MIT and the mission of the NIEHS Superfund Research Program.

As part of the MIT SRP, the Kroll Lab at MIT is working to better understand what happens to PAHs during their chemical transformations in the atmosphere. This is important because PAHs are known to have negative impacts on human health, and so better understanding of their evolution in the atmosphere will inform how long they pose a threat after emission. Importantly, some reactions in the atmosphere produce reaction products that can be equally or more harmful than the parent PAH,¹ yet these reaction products are not currently monitored. By studying PAH evolution over time, harmful byproducts of atmospheric reactions can be identified and suggested for routine monitoring.

Studying the evolution of PAHs in the atmosphere first requires an understanding of PAH emission. PAHs can be emitted in both the gas and particle phase depending on the size of the compound. PAHs in both phases are important, but larger PAH molecules mainly reside in the particle phase and are typically more harmful to human health than gas phase PAHs.² For this reason, the work of the MIT SRP is mainly focused on particle phase PAHs. Once in the atmosphere, there are three predominant processes that affect their evolution, illustrated in Figure 2: 1) reaction with UV light produced by the sun, 2) reactions between particle phase PAHs and gas phase oxidants (i.e., heterogeneous reactions), and 3) the condensation of gas phase reactions. The relative importance of these pathways is determined by how quickly the process degrades the parent PAH and the types of reaction products that are formed. Fast degradation pathways of the parent PAH will decrease the health risk if the products formed from reaction are innocuous, but if the products formed are more harmful than the parent compound the reaction pathway becomes detrimental.

In order to determine which process is most impactful on human health, we have developed an experimental setup that can generate particle phase PAHs and simulate the three reaction pathways. Researchers measure PAH evolution using an Aerosol Mass Spectrometer (AMS) and Proton Transfer Reaction Mass Spectrometer (PTR) that can measure the concentration of individual PAHs and reaction products in the particle phase over time. To date this setup has been used to study heterogeneous reactions between particles and gas phase oxidants. Particle phase perylene, a representative PAH, has been created and reacted with two common atmospheric oxidants, hydroxyl radical (OH˙) and ozone (O₃). Figure 3 shows the degradation of perylene by hydroxyl radical and ozone over time, and Figure 4 shows the reaction products products that are formed during perylene degradation by hydroxyl radical over time.

The reactions can be compared using the two factors mentioned previously, decay of parent PAH and formation of products. Results from the AMS show that the reaction of perylene with O₃ in the atmosphere is initially faster than OH˙, but levels off after around one day of exposure while the reaction with OH˙ keeps going (Figure 3). This could mean that PAHs emitted in areas where heterogeneous reaction with O₃ is the only process, only around 30% of the PAH will go away and the rest will remain a risk for human exposure. In terms of reaction products, similar products are formed in both reactions, but the products are formed more quickly from reaction with OH˙. This could mean that for PAHs that spend less than a day in the atmosphere, the products from reaction with OH˙ are of most concern to human health.

Future work on this project will include extending the experimental setup to other atmospheric processes and testing additional PAHs. This will begin to inform what processes are most important as well as identify potentially harmful byproducts. Beyond this, we aim to identify which chemical species of PAHs and degradation products have impacts on health. Collaboration with Projects 3, 4, and 5 of the MIT SRP will help identify the health risks of PAHs and their byproducts so that harmful ones can be suggested for routine monitoring.

[1] Finlayson-Pitts, B. J., & Pitts Jr, J. N. (1999). Chemistry of the upper and lower atmosphere: theory, experiments, and applications. Elsevier.

[2] Calvert, J. G., Atkinson, R., Becker, K. H., Kamens, R. M., Seinfeld, J. H., Wallington, T. H., & Yarwood, G. R. E. G. (2002). The mechanisms of atmospheric oxidation of the aromatic hydrocarbons. Oxford University Press.