Human health metrics are often overlooked in the analysis of alternative energy sources. Part of the reason for this is that, of the three classes of inputs surveyed here, these tend to have the highest embedded uncertainty. The exposure to hazard­ous substances varies significantly, and this can greatly impact the results of an analysis. Further, since limited toxicological data is available for many compounds, developing reliable causal relationships is a challenge. When impacts can be quantified in a life cycle context, they are often reported in terms of disability — adjusted life years.

Ignoring the contribution that human-health indicators may have on algae-to — energy life cycle studies could be an important oversight for several reasons. The most dangerous substances on the United States Environmental Protection Agency list of carcinogenic chemicals reveals that many are agricultural chemicals. If algae are deployed as an alternative to terrestrial agriculture, which is heavily reliant on harmful herbicides, fungicides, and pesticides, there could be a net advantage to adopting aquatic species for biomass generation. Of course, the algae cultivation sector is too young to know whether it will require significant flows of agricultural chemicals to cope with pests or other problems. Similarly, the water quality implica­tions of large-scale algae cultivation could have mixed impacts. On the one hand, algae could remove contaminants from water sources, serving effectively like a large ecosystem-level “liver” for toxin removal. On the other hand, algae could excrete low levels of toxic chemicals as exemplified by coastal red tides. In short, any large-scale production of algae is likely to have some human health conse­quence and even though it is difficult to predict how those will manifest at this early stage, it is not difficult to anticipate that better tools will be needed to understand these relationships as the technology matures and becomes deployed.