The results obtained in this study suggest a serious bias in publications on environmental chemicals. While regulatory agencies request information on poorly studied, potentially serious environmental hazards, the publications in scientific journals emphasize a relatively small, selective number of highly prominent chemicals, about which large numbers of articles were published already during the past century. The focus on inorganic substances is noteworthy; the top-10 substances are all metals or metalloids. Copper is a well-established ecological hazard, and lead toxicity has been known since antiquity. As also indicated by the Web of Science search, all the most popular chemicals had substantial publication numbers also in the past century. All told, tens of thousands of articles have been published in scientific journals on these substances.
While the well-known chemicals remained a focal point in published research reports, the substances from the EPA priority list attracted only a small number of publications, if any. The long-term prominence of substances commonly covered in articles in environmental science journals therefore ignores the needs of regulatory agencies like the U.S.EPA. The enormous contrast in publication coverage suggests that regulatory needs for documentation are not impacting environmental science priorities.
The retained focus on a limited number of prominent chemicals may be due to continued environmental or occupational exposures, in spite of any preventive efforts [12
]. Although that may be true for many of the commonly studied chemicals, especially those that are persistent in the environment and in the human body, the question is whether incomplete or lack of intervention should justify research focus on issues that have already been thoroughly covered by extensive research. A key question is whether the desire for a solid proof on certain chemicals should overrule the need for basic information on other potential hazards. Still, some well-studied substances may represent an important scientific paradigm or useful reference, thereby justifying their inclusion in some research studies. We did not attempt to evaluate the justification of the focus in individual studies, nor did we attempt to assess the validity or impact of individual articles.
This study relies on bibliographic databases being routinely available to obtain information on the coverage of chemicals by scientific journals in the environmental science field. Because of the reliability of the data bases used, the results are likely to represent overall tendencies of scientific attention to different chemical compounds. However, each CAS number link may refer to an article with only limited measurement data or, on the other extreme, a comprehensive review. The depth of our study is based solely on the mere number of articles located. While SciFinder is unlikely to underestimate the numbers of publications, unless a substance is commonly related to more than one CAS number, e.g., due to isomers, impurities, or mixtures, the numbers obtained from our Web of Science title searches are most likely underestimates, as all relevant articles may not have included the name of the substance in the title of the publication. Assuming that this error would be unlikely to change much over time, the comparisons of relative numbers before and after 2000 would still be representative.
A previous study on European environmental research listed as many as 711 journals [9
] publishing in the field of environmental research. However, most of these journals must have published only a small number of the 6329 references identified, and only a minority would have dealt with environmental chemicals. Also, some general science or medicine journals occasionally publish articles on environmental chemicals, and these publications were not included in the searches based only on the major 78 environmental and toxicology journals. Thus, while the total number of articles covered by the journals included in the present study was probably underestimated, it is unlikely that any serious bias would have occurred by our selection of topic-specific journals.
A certain amount of inertia in regard to the choice of research topics is undoubtedly present at universities, among researchers, and funding agencies [14
]. Tradition and availability of established methods, existing instrumentation, and experience no doubt play a role. The metals, for example, can be easily and inexpensively measured by a common instrument called an atomic absorption spectrometer. Modern equipment, such as inductively-coupled plasma mass spectrometry, can even measure several metals at the same time. Likewise, some of the tar chemicals can be measured by gas chromatography, another inexpensive instrument that is widely available in research laboratories worldwide. Hundreds of analyses can be produced in a week, perhaps enough to justify a manuscript for a scientific journal. When inexpensive routine methods are available, who can blame researchers for taking advantage of generating another research report?
If feasibility is a criterion, then the choice is obvious. Thesis advisors, who have themselves studied metals, and who have established methods for their analysis, will likely recommend metals research to their students, rather than a more cumbersome project on lesser known environmental chemicals, where the outcome may be difficult to predict.
Within scientific communities focusing on heavy metals, aromatic hydrocarbons, or chlorinated organic compounds, citations are likely to be high, to the extent that members of these communities are highly active, participate in specialized conferences, and publish frequently. They are likely to cite their own work and the publications by close colleagues that support their research perspective. Some support for this notion is apparent from Table , where arsenic and biphenyls have twice the number citations of aluminum. Coverage in high-impact journals with a wide readership potentially will lead to these articles being more likely to be cited [15
], as opposed to those of less prominence, thereby perpetuating the assumed importance of the research. Again, arsenic and biphenyls were more frequently covered in the upper tertile of environmental science journals, but this tendency was not apparent for other environmental chemicals.
] described the self-prophetic bias that maintains a continued prominence of a small number of scientists and their publications. He dubbed this a 'Matthew' effect, referring to the New Testament ('For unto every one that hath shall be given, and he shall have abundance: but from him that hath not shall be taken away even that which he hath'). An extension of this principle assigns importance to widely published topics, in our case specific environmental chemicals, for the very reason that they are already widely covered in the literature. Whether or not they are persistent in the environment or the human body, they become persistent in the scientific literature. The tens of thousands of articles on copper, lead, cadmium, and other prominent environmental hazards testify to the enormous investments in studying, reporting, and publishing on these popular substances. In other words, fame reinforces itself [11
Such self-affirming, or perhaps self-serving, bias can impede the progress of research in new fields and studies covering new substances, thereby resulting only in incremental advances, at best, from the research investments. The EPA priority chemicals continue to be ignored, and scientific discovery is impeded by a narrow focus on a limited number of favored chemicals. As noted by Merton [16
], a disproportionate focus on prominent substances in environmental science journals may be nurtured by the self-prophecy associated with an increased number of citations in the scientific literature.
Several factors likely play a role in addition to the motivation of researchers to tackle challenging environmental health problems. Apart from mere inertia within the research institutions, a major determinant is the accessibility of federal, private or other funding for research in the less apparent and only suspected hazards. Some degree of inertia probably also exists within funding agencies. Research on major environmental chemicals is easier to evaluate due to the availability of experts. They may tend to recommend that further research on their favorite substances is warranted, one possible advantage being that additional studies will generate more attention and perhaps citations of their own publications. For the agencies, an attractive aspect is that the research outcome is easier to predict and to interpret as well as probably being less expensive. If focusing on poorly-known substances, the risk is greater that the detection method is problematic, the doses used are inappropriate, or that some other, unexpected problem will render the results invalid or uninformative. However, for these very reasons, research on less-commonly studied chemicals is highly needed, not the least to pave the way for more definite studies and to facilitate innovation and unexpected discoveries.