ncasi March 7, 2013

1

The Significance of Dibenzofuran in Creosote and Creosote-Treated Railroad Ties 

This explanatory document has been prepared in response to USEPA’s request for additional information 

regarding (a) the presence of dibenzofuran in creosote and creosote-treated wood, and (b) whether the 

presence of dibenzofuran can indicate the concurrent presence of the polychlorinated versions of these 

compounds, viz., polychlorinated dibenzo p-dioxins and dibenzofurans (PCDD/F – often collectively 

termed dioxins). As discussed in the sections below, dibenzofuran (a) is an un-substituted heterocyclic 

organic compound with two benzene rings and one fused furan ring, (b) is distinctly different from the 

polychlorinated versions of the compound (reportable dioxins/furans in Table 1 below) – the latter are 

formed as a result of various degrees of substitution of the hydrogen atoms in dibenzofuran with chlorine, 

and (c) has a unique CAS number that is different from the CAS numbers for the reportable

dioxins/furans. Therefore, the presence of dibenzofuran does not automatically translate into the presence 

of reportable dioxins/furans. 

Background

In the preamble of the February 7, 2013 Commercial and Industrial Solid Waste Incineration Units: 

Reconsideration and Final Amendments; Non-Hazardous Secondary materials That Are Solid Waste; 

Final rule (see page 9173) EPA lists “dioxins” in addition to “dibenzofurans” as contaminants known to 

be present in creosote-treated railroad ties. EPA’s listings appear to be based on 

an IP (International Paper) railroad tie sample from 1989/1990 contained nearly 900 ppm dibenzofuran

and that creosote can contain up to 4.5% of dibenzofuran. However, no data exists on the class of 

dioxins that would include 2,3,7,8-tetrachlorodibenzo-p-dioxin or any other dioxins. In discussions with 

EPA staff, it became clear that an assessment of why “dioxins” would not form or be present in creosote 

wood would help justify why testing for dioxin rather than just dibenzofurans would not be necessary. 

Dibenzofuran is a distinctly different compound from PCDD/F 

The reportable form of dioxins/furans, in reality, represents the 17 specific isomers of dioxins and furans, 

7 polychlorinated dibenzo-p-dioxins or PCDDs and 10 polychlorinated dibenzofurans or PCDFs, each of 

which has a specific CAS No. as shown in Table 1 below.

Dibenzofuran, (CAS No. 132-64-9), on the other hand, is a heterocyclic organic compound with 

the chemical structure shown below. It is an aromatic compound that has two benzene rings fused to 

one furan ring in the middle. All of the numbered carbon atoms have a hydrogen atom bonded to each of 

them (not shown in the image). Dibenzofuran is an aromatic ether having the chemical formula C12H8O.
ncasi March 7, 2013

2

Table 1. Reportable “Dioxin and dioxin-like Compounds”

CAS NO. COMPOUND

ABBREVIATED 

NAME

1746-01-6 2,3,7,8-tetrachlorodibenzo-p-dioxin 2,3,7,8-TCDD

40321-76-4 1,2,3,7,8-pentachlorodibenzo-p-dioxin 1,2,3,7,8-PeCDD

39227-28-6 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin 1,2,3,4,7,8-HxCDD

57653-85-7 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin 1,2,3,6,7,8-HxCDD

19408-74-3 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin 1,2,3,7,8,9-HxCDD

35822-46-9 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin 1,2,3,4,6,7,8-HpCDD

3268-87-9 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin 1,2,3,4,6,7,8,9-OCDD

51207-31-9 2,3,7,8-tetrachlorodibenzofuran 2,3,7,8-TCDF

57117-41-6 1,2,3,7,8-pentachlorodibenzofuran 1,2,3,7,8-PeCDF

57117-31-4 2,3,4,7,8-pentachlorodibenzofuran 2,3,4,7,8-PeCDF

70648-26-9 1,2,3,4,7,8-hexachlorodibenzofuran 1,2,3,4,7,8-HxCDF

57117-44-9 1,2,3,6,7,8-hexachlorodibenzofuran 1,2,3,6,7,8-HxCDF

72918-21-9 1,2,3,7,8,9-hexachlorodibenzofuran 1,2,3,7,8,9-HxCDF

60851-34-5 2,3,4,6,7,8-hexachlorodibenzofuran 2,3,4,6,7,8-HxCDF

67562-39-4 1,2,3,4,6,7,8-heptachlorodibenzofuran 1,2,3,4,6,7,8-HpCDF

55673-89-7 1,2,3,4,7,8,9-heptachlorodibenzofuran 1,2,3,4,7,8,9-HpCDF

39001-02-0 1,2,3,4,6,7,8,9-octachlorodibenzofuran 1,2,3,4,6,7,8,9-OCDF

Dibenzofuran

When the hydrogen atoms in positions 1, 2, 3, 4, 6, 7, 8 and 9 in the dibenzofuran molecule (see figure) 

are substituted with chlorine, this results in one of the 10 polychlorinated dibenzofurans listed in Table 1. 

Thus, the molecule dibenzofuran is not the same as polychlorinated dibenzofurans or PCDFs, and has its 

own CAS No. (132-64-9). In fact, wikipedia (http://en.wikipedia.org/ wiki/Dibenzofuran) makes specific 

mention of the following inaccurate characterization:

“Dibenzofurans can also inaccurately refer to polychlorinated dibenzofurans (PCDFs), a family of 

organic compounds with one or several of the hydrogens in the dibenzofuran structure replaced by 

chlorines. For example, 2,3,7,8-tetrachloro dibenzofuran (TCDF) has chlorine atoms substituted for each 

of the hydrogens on the number 2, 3, 7, and 8 carbons (see structure below). Polychlorinated 

dibenzofurans are much more toxic chemicals than the parent compounds with properties and chemical 

structures similar to polychlorinated dibenzodioxins. These groups together are often inaccurately 

called dioxins.”
ncasi March 7, 2013

3

2,3,7,8-Tetrachlorodibenzofuran

The general structure for PCDFs is as follows

Thus, while “creosote” used in preparing railroad ties may indeed have high levels of dibenzofurans (up 

to 4.5% or 45,000 ppm), and the dibenzofuran concentration in one RR sample treated with creosote was 

measured at ~ 900 ppm, this compound is just an organic and should not be confused with dioxins or 

furans, which are clearly multiple chlorine-substituted forms of dibenzofurans and dibenzodioxins (the 

latter has the same structure as PCDFs except the two benzene rings are fused to one dioxin ring in the 

middle as shown below).

The general structure for PCDDs is as follows

Are dioxins formed either during creosote application or creosote manufacture?

As discussed earlier, dibenzofuran is neither a chlorinated dioxin nor a chlorinated furan, and the terms 

“dioxins” or “PCDD/Fs” are specifically reserved for 17 specific chlorinated dioxins and furans each with 

its own CAS numbers. EPA also requested information on whether “dioxins” can be formed either 

during the manufacture of creosote (mainly from coal tar) itself or during the application of creosote to 

railroad ties (for preservation). 

The second aspect, viz., the potential for dioxin formation during the application process of creosote to 

railroad ties, can be readily dismissed as reaction mechanisms other than some form of combustion or the 

bleaching of pulp with chlorine have never been reported. Trace levels of dioxins, especially the octachlorinated types (OCDD and OCDF), are ubiquitous in nature. Trace amounts (mainly OCDFs) have 
ncasi March 7, 2013

4

been detected in ground and river water, and even in virgin wood and bark. The phenomenon of poor

combustion of biomass during forest fires and other non-industrial biomass burning, which results in 

OCDD/Fs being continually formed and re-deposited on the forest floor, is generally assumed to be origin 

of these OCDD/Fs in nature.

There is also no evidence in published literature on PCDD/F formation during the manufacture of 

creosote. Creosote applied to railroad ties is manufactured predominantly by distilling coal tar, a 

byproduct from the coking plants of steel mills (http://apps.kemi.se/flodessok/floden/kemamne_ 

eng/kreosot_eng.htm). It is made from the “medium” distillates of coal tar that have boiling points in 

excess of 200C. Occasionally, there is discussion on “dioxins” in the context of remediation of wood 

preservation sites. However, this is invariably in the context of pentachlorophenol (PCP) that is also used 

by wood preservers besides creosote. Dioxins are believed to be a common impurity in commercial-grade 

PCP (http://www.epa.gov/region4/superfund/sites/npl/florida/amercreopenfl.html.