In my reading on polycyclic aromatic hydrocarbons (PAHs), many of the studies from a few decades ago focused on figuring out where they were coming from and if they posed a risk. Then, in the mid to late 1990s, I saw a shift toward looking for ways to limit exposure. What’s the upshot of that earlier research? PAHs are persistent, scary, and connected to combustion. As for risk limitation, we’ve got more options with some compounds than with others, so limiting emissions to start with is probably key.
Some PAHs are created by industrial combustion. Photo courtesy of USFWS
What are PAHs in the first place? As I mentioned in my last post, there are a variety of different kinds, but they all have multiple rings of carbon and hydrogen atoms. The ones that we are most concerned with here, like fluorene, naphthalene, and pyrene, can be produced by the burning of fossil fuels, locations with very high temperatures, forest fires, tobacco smoke, and other situations where organic materials are combusted but particles are left behind (Blumer & Youngblood 1975). [Given that description, you might think that pollution from vehicles is important, and it is, but a 2003 study in China found that, although traffic emissions were roughly twice as plentiful as emissions from restaurant and home cooking, the cooking particles were 11 times as potent (Li et al. 2003)] These particles come into contact with us when we inhale them or they settle into the water we drink and the soil that nourishes the foods we eat- that’s where things pick up some speed. PAHs are not active mutagens and carcinogens on their own, but mammalian metabolisms (like ours) convert these substances into reactive derivatives, for example with liver enzymes, and then the now-carcinogenic hydrocarbons get to work targeting DNA, RNA, and proteins (Pashin & Bakhitova 1979). PAH exposure has been linked to liver lesions in fish (Landahl et al. 1990), lung cancer in humans (Sherson et al. 1990), and other diseases (Pashin & Bakhitova 1979).
Why are they persistent? According to Genney et al. (2004), PAHs are good at sticking to soil particles and bad at dissolving in water, so they aren’t in easy access for soil fungi to degrade. Some PAHs are very volatile, so they tend to vaporize quickly which means that they don’t stick around long enough in one place for degradation (Joyce et al. 1998). What can we do for clean up? Joyce et al. (1998) did find that some PAHs, such as pyrene and phenanthrene, were degraded by microbes found in compost, and Hammel (1995) noted that the same fungi which causes white-rot in wood also degraded phenanthrene and other PAHs. A 2001 study also found that a new design for iron and PAH biofiltration from groundwater successfully degraded PAHs with 2 and 3 rings of atoms, but not those with 4 rings (Richard & Dwyer 2001). As I said above, it’s probably good to limit emissions in the first place.
So we have a sense of where these hydrocarbons come from, why they present a risk for us, and how we can approach the issue of remediation. What new information have we gained in the last 10 years or so? That’s what I’ll look at in my next post.
Works Cited:
Blumer, M and WW Youngblood. 1975. Polycyclic aromatic hydrocarbons in soils and recent sediments. Science 188: 53-55.
Genney, DR, Alexander, IJ, Killham, K and AA Meharg. 2004. Degradation of the polycyclic aromatic hydrocarbon (PAH) fluorene is retarded in a scots pine ectomycorrhizosphere. The New Phytologist 163: 641-649.
Hammel, KE. 1995. Mechanisms for polycyclic aromatic hydrocarbon degradation by ligninolotic fungi. Environmental Health Perspectives 103: 41-13.
Joyce, JF, Sato, C, Cardenas, R and RY Surampalli. 1998. Composting of polycyclic aromatic hydrocarbons in simulated municipal solid waste. Water Environment Research 70: 356-361.
Landahl, JT, McCain, BB, Myers, MS, Rhodes, LD and DW Brown. 1990. Consistent associations between hepatic lesions in English sole (Parophrys vetulus) and polycyclic aromatic hydrocarbons in bottom sediment. Environmental Health Perspectives 89: 195-203.
Li, C-T, Lin, Y-C, Lee, W-T and P-J Tsai. 2003. Emission of polycyclic hydrocarbons and their carcinogenic potencies from cooking sources in the urban atmosphere. Environmental Health Perspectives 111:483-487.
Pashin, YV and LM Bakhitova. 1979. Mutagenic and carcinogenic properties of polycyclic aromatic hydrocarbons. Environmental Health Perspectives 30: 185-189.
Richard, DE and DF Dwyer. 2001. Aerated biofiltration for simultaneous removal of iron and polycyclic aromatic hydrocarbons from groundwater. Water Environment Research 73: 673-683.
Sherson, D, Sabro, P, Sigsgaard, T, Johansen, F and H Autrup. 1990. Biological monitoring of foundry workers exposed to polycyclic aromatic hydrocarbons. British Journal of Industrial Medicine 47: 448-453.