“Unknown Knowns” and
Precautionary Principle
the
A New Taxonomy for Risk Assessment
BY FRANK MIRER
“There are known knowns; there are things we know that we know. There are known unknowns; that is to say, there are things
that we now know we don’t know. But there are also unknown unknowns; there are things we do not know we don’t know.”
—Former U.S. Secretary of Defense Donald Rumsfeld
While Donald Rumsfeld and I don’t often agree, I find his
“known knowns,” “known unknowns,” and “unknown un-
knowns” to be an effective taxonomy for risk assessment. After
spending a weekend at the Collegium Ramazzini meeting—a
hotbed of fellow precautionary principle advocates, REACH en-
thusiasts and nanotechnology distrusters—I’d create an addi-
tional category for “unknown knowns” in occupational health.
Unknown knowns, or maybe ignored knowns or forgotten
knowns, are the material results (which could support rulemak-
ing) recognized by authoritative bodies but forgotten or ignored
by the IH and public health communities.
The Rumsfeldian Taxonomy
For practitioners, it’s a known known that, with the exception of
16 substances, OSHA has not promulgated permissible exposure
limits (PELs) based on post-1970 science and policy assumptions. For several dozen substances, a significant risk to workers
exposed at the PEL is also a known known.
The known unknown is the number of workers likely to be
exposed at levels lower than the PEL, but which pose a significant risk. Knowing this answer would establish priorities for setting new PELs.
The notion of unknown knowns is applicable to nanotechnol-
ogy. Carbon black (a low-tech version of carbon nanotubes) and
nano-sized titanium dioxide are clearly carcinogenic in labora-
tory studies. Diesel particulate matter, a nanoparticle aerosol, is
“probably” carcinogenic to humans according to the Interna-
tional Agency for Research on Cancer (IARC). Yet the statements
of public health advocates on the need for “precautionary” ap-
proaches to nanoparticles undermine the fact that scientifically
justified exposure limits, or reference concentrations factors of
10 below the canonical 5 mg/m3 limit for nuisance dust, could
be set now. Personally, I’d start at 0.100 µg/m3 for nano tita-
nium dioxide and go down from there. Profoundly greater po-
tency associated with nanoparticle form may emerge later, but
intermediate action could be started today.
IARC’s Knowns and Unknowns
IARC recently published two important, authoritative listings of
knowns and unknowns for “priority” carcinogens:
A Review of Human Carcinogens, Volume 100, is available in
summary at http://monographs.iarc.fr/ENG/Meetings/index1.php
(log in to The Lancet Oncology to get the summaries without
charge). Six working groups updated earlier monographs, including two groups dealing with multiple agents found in the
occupational environment. Their reports affirmed existing
human carcinogens (IARC Group 1) and added anatomical sites
with sufficient evidence. Their most notable conclusion is that
sufficient evidence exists to associate cancer of the larynx and
ovaries with asbestos exposure, and leukemia with formaldehyde
exposure.
IARC Publication 42, “Identification of research needs to resolve the
carcinogenicity of high priority IARC carcinogens,” was initiated by
NIOSH in collaboration with IARC, the National Institute of Environmental Health Sciences (NIEHS), the American Cancer Society (ACS),
and the National Cancer Institute (NCI). The publication included a
selection of 20 agents, previously reviewed in IARC monographs,
based on their potential for workplace or environmental exposure
and their importance or interest by a particular agency. The purpose was to identify research needed to develop a more definitive
classification and to help funding agencies determine possible
