The precautionary principle and environmental persistence: prioritizing the decision-making process

The precautionary principle and environmental persistence:prioritizing the decision-making process

Todd Gouin *

Safety and Environmental Assurance Centre, Unilever, Colworth Science Park, Sharnbrook, Bedfordshire MK44 1LQ, UK

e n v i r o n m e n t a l s c i e n c e & p o l i c y 1 3 ( 2 0 1 0 ) 1 7 5 – 1 8 4

a r t i c l e i n f o

Published on line 25 February 2010

Keywords:

Precautionary principle

Persistent

Bioaccumulative

Toxic substances

REACH

Decamethylpentasiloxane

a b s t r a c t

There is continuing activity among regulatory bodies to assess and prioritize chemicals used

in commerce based on their potential to be persistent, bioaccumulative, and toxic (PBT).

Reliable data needed to perform a PBT hazard or risk assessment, however, may not always

be readily available. Consequently concern may arise regarding the potential for false

positives and false negatives to be wrongly classified. In order to more effectively classify

substances, adequate time is needed to acquire the necessary data to support the overall

PBT assessment. Of particular interest is the question of whether or not restrictions on the

use and manufacture of a substance can be delayed to allow time to conduct the necessary

field and laboratory studies of a particular substance? To address this question it is

demonstrated that chemical partitioning property and environmental persistence informa-

tion can be effectively combined to provide guidance for regulatory priority setting. Specifi-

cally, it is argued that substances that have media specific half-life values that exceed the

regulatory threshold value for persistence under the EU chemicals REACH program, for

example, are more likely to have a ‘legacy’ associated with their use when the log KOA > 8,

and when they are emitted to air or soil. Thus, precautionary actions limiting the use and

manufacture of the substance may be warranted. Whereas substances emitted to air with

log KOA < 6 and log KAW > �2 are less likely to have a ‘legacy’ associated with their use. Thus

precautionary actions in the absence of data may not be warranted.

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1. Introduction

Identifying and managing substances of very high concern

(SVHC) used in commerce in order to protect humans and the

environment is a primary objective of the parties of the

Stockholm Convention on persistent organic pollutants

(POPs), the European regulation REACH (Registration, Evalua-

tion and Authorization of Chemicals), and the Chemicals

Management Plan in Canada (UNEP, 2001; Council EU, 2006;

Ambrose and Clement, 2006). It is notable that each of these

regulatory instruments incorporates a form of the ‘‘precau-

tionary principle’’ as a potential tool to assist in the

* Tel.: +44 1234264770.E-mail address: [email protected].

1462-9011/$ – see front matter # 2010 Elsevier Ltd. All rights reserveddoi:10.1016/j.envsci.2010.01.005

management of SVHC (Maguire and Ellis, 2005; Pelley, 2001;

Scheringer et al., 2006; Davies, 2006; Applegate, 2008). The

implication is that precautionary measures can be imple-

mented in instances where there is limited data for a

substance and for which concern regarding its environmental

fate and potential toxicity has been raised. Thus the capacity

to act in the absence of data to prevent the potential for

irreversible harm can be perceived as prudent (Sandin et al.,

2004; Karlsson, 2006; Berg and Scheringer, 1994; Godduhn and

Duffy, 2003). Conversely, adoption of the precautionary

principle may be perceived negatively (Scheringer et al.,

2006; Morris, 2002; Holm and Harris, 1999). For instance,

.

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http://dx.doi.org/10.1016/j.envsci.2010.01.005


precautionary actions taken to restrict the use of a chemical

that is wrongly identified as PBT (false positive) may result in

negative consequences to society as a result of losing an

otherwise important chemical substance (Klecka et al., 2009).

Consequently, reducing both Type I and Type II errors, and

creating more effective methods to identify and manage

chemicals of potential concern, has been the subject of a

number of recent studies (Arnot and Mackay, 2008; Muller-

Herold et al., 2005; Brown and Wania, 2008; Czub et al., 2008;

Muir and Howard, 2006; ECETOC, 2005; Cowan-Ellsberry et al.,

2009; van Wijk et al., 2009).

Currently, identifying substances as potential PBT or POP

candidates relies on determining if specific properties of a

substance exceed a threshold value (Arnot and Mackay, 2008;

Klecka et al., 2009). The screening criteria used under various

regulatory instruments can be found in Muir and Howard

(2006) or Boethling et al. (2009). Thus, to adequately assess the

risk profile of a POP or PBT candidate substance, it is essential

that quality assured monitoring data, obtained from robust

studies made in both the field and laboratory, are obtained

(Swackhammer et al., 2009). Additionally, the use of environ-

mental fate and exposure models as part of the regulatory

process has become increasingly important, complementing

the interpretation of information obtained from monitoring

data in a manner that is generally believed to help reduce

uncertainty associated with PBT or POP assessment (Stroebe

et al., 2004; Fenner et al., 2006; Zachary and Greenway, 2009;

Wegmann et al., 2009; Scheringer et al., 2009; Cowan-Ellsberry

et al., 2009; van Wijk et al., 2009). Of particular interest is the

recent development of the Organization for Economic Coop-

eration and Development (OECD) Screening Tool for overall

environmental persistence (POV) and long-range transport

potential (LRTP), which has been recommended as a tool to

assist regulators in their capacity to assess the hazard criteria

of P and LRTP (Wegmann et al., 2009).

The complementary nature by which monitoring data and

models are being used is promising, and it provides a

foundation upon which precautionary actions, if needed,

can be based upon (Muller-Herold et al., 2005; Sandin et al.,

2004). Identifying the potential for a substance to be persistent

based on monitoring data alone, for instance, can cause

regulatory actions to be delayed, while the necessary data are

accumulated. Models, however, have the capacity to estimate

persistence earlier in the assessment, and when complemen-

ted with monitoring data strengthen the basis for implement-

ing precautionary actions (Muller-Herold et al., 2005; Sandin

et al., 2004).

An interesting attribute of environmental persistence, is

that it provides an opportunity for assessing the environmen-

tal response time a chemical substance may have to emissions

reductions. Thus an estimate of the lag-time (i.e. the time

between when emissions reductions begin and when envi-

ronmental levels decline to a specified level) that a substance

may have in the environment can be obtained. A recent

example of such an approach can be seen in the study by

Gouin and Wania (2007), who demonstrate the relationship

between the physical–chemical properties of a substance and

the potential for a lag-time between emissions reductions and

Arctic contamination potential (ACP). ACP describes the

relative potential for a substance to accumulate in the Arctic,

and can be used as a combined metric for long-range transport

and persistence, with an emphasis on the Arctic environment

(Wania, 2006). Similarly, Mackay and Reid (2008) argue that the

concept of a distant residence time (DRT) could be used to

assess the efficacy of regulatory measures at reducing

environmental levels.

In this study it is suggested that prioritization of the

precautionary decision-making process for a potential PBT or

POP substance is a function of its overall environmental

persistence, and thus corresponds to the physical–chemical

properties of the substance. This is explored based on the

output of a number of environmental fate models.

2. Methods

2.1. Estimating P and LRTP metrics

The consensus of an expert group on multimedia models

involved in developing the OECD Tool was a recommendation

to estimate an overall environmental persistence (POV), as a

metric for P, and the characteristic travel distance (CTD) as

described by Beyer et al. (2000) and transfer efficiency (TE) as

described by Macleod and Mackay (2004), as metrics for LRTP

(Wegmann et al., 2009). The use of POV estimated from an

environmental fate model is recommended since it is believed

that it will provide a better representation of the hazard

associated with P (Webster et al., 1998; Arnot and Mackay,

2008). This is because the approach accounts for the

partitioning properties of the substance in relation to its

reactivity, information not considered when using a media

specific half-life approach. In the OECD Tool calculations of

POV, CTD, and TE are performed for a chemical emitted into a

steady-state multimedia environment, consisting of air,

water, and soil, and values can be plotted against a reference

value of 195 d for POV, 5100 km for CTD, and 2.2% for TE

(Wegmann et al., 2009). The CTD estimates the LRTP of a

substance by calculating the distance from between the point

of release of the chemical, to the point where the concentra-

tion has dropped by 1/e, i.e. 37%, of its initial value. Whereas,

the TE of a substance is estimated as the ratio of the net

deposition mass flux from air to surface media in a region

adjacent to the region where the chemical is released, and the

mass flux of the chemical emitted to air in the release region.

The OECD Tool default values have been used as a reference

point based on the results for well-established SVHC, although

it is important to note that these values are not used for

regulatory purposes, unlike the persistence criteria defined

under REACH (Scheringer et al., 2009). The unit world

environment has been developed to represent the global land

to water surface ratio (30% land and 70% water) (Wegmann

et al., 2009). It is notable that the global parameterization

characterized in the OECD Tool may not reflect the environ-

mental fate of chemicals emitted to other geographic scales,

such as regional or country-specific environments. Thus the

use of the OECD Tool provides an illustrative example of the

approach developed. It is believed that it would be possible to

adopt the approach presented in this study using the OECD

Tool to region-specific environments in the future. Model

parameterization details of the OECD Tool are provided by

e n v i r o n m e n t a l s c i e n c e & p o l i c y 1 3 ( 2 0 1 0 ) 1 7 5 – 1 8 4 177

Wegmann et al. (2009). Briefly, the values reported in the OECD

Tool for POV are those that have been estimated for the steady-

state residence time with the highest value, based on results

obtained from three different emission scenarios, i.e. to air,

water, and soil (Wegmann et al., 2009). Similarly, the CTD

value reported represents the higher CTD value between the

results obtained from emission to air and water.

In addition to the OECD model output, ACP values are also

calculated using the GloboPOP model, as described by Wania

and Mackay (1995). The eACP10 provides an indication of a

chemicals potential to contaminate remote regions, such as

the Arctic, based on the ratio of the amount present in Arctic

surface media to the cumulative amount emitted globally after

10 years of steady-state emissions, as described by Wania

(2003, 2006).

2.2. Selection of illustrative chemicals

Activity under the Stockholm Convention on POPs, REACH,

and Canada’s Chemical Management Plan is aimed at

identifying SVHC. Under Canada’s Chemical Management

Plan, for instance, several substances have recently been

identified as chemicals of concern, and for which high

priority has been assigned towards assessing their potential

risks to the environment and human health. Notable among

these substances of interest has been the recent high profile

assessments of bisphenol A, perfluoroctane sulfonate

(PFOS), phthalates, and polybrominated diphenyl ethers

(PBDEs). Many of these substances have also been scruti-

nized by other regulatory bodies, such as the Stockholm

Convention, which has recently added nine substances,

including the PBDEs and PFOS, to its initial list of 12 POPs,

and has recently reviewed three substances as potential

candidates: hexabromocyclododecane (HBCDD), short

chained chlorinated paraffins, and endosulfan. Additionally,

the recent implementation of REACH has resulted in five

substances being added to its candidate list of SVHC for

authorization based on their meeting criteria as PBT

substances; tributyltin oxide (TBTO), xylene musk, HBCDD,

anthracene, and short chain chlorinated paraffins (ECHA,

2010). Therefore a short-list of substances consisting of the

five PBT substances listed under REACH, three Stockholm

Convention POPs, which are also included on the Canadian

list of chemicals of concern, and two substances of emerging

concern, the organosiloxane, decamethylcyclopentasiloxane

(D5), and the current-use pesticide, endosulfan have been

selected as illustrative examples. Additionally, three sub-

stances included under the Montreal Protocol on substances

that deplete the ozone layer, and five high production

volume chemicals (HPVCs) used in the United States have

been included. The properties for the substances used in the

various models are listed in Table 1.

2.3. Chemical space diagrams of POV and lag-times

The criteria used to assess substances as persistent differs

slightly between the various jurisdictions (see Muir and

Howard, 2006). For instance, persistence is characterized in

Canada as a substance that has a half-life in air, water, soil or

sediment that exceeds 2 d, 182 d, 182 d, and 365 d respectively,

whereas under REACH a substance will be prioritized as

persistent if it has a half-life in fresh water or sediment that

exceeds 40 d or 120 d, respectively, or in marine water or

sediment that exceeds 60 d or 180 d, respectively (Muir and

Howard, 2006). In addition, under REACH a substance may also

be classified as very persistent (vP) if it has a half-life in either

fresh or marine water that exceeds 60 d, or a half-life in either

fresh or marine sediment that exceeds 180 d (Muir and

Howard, 2006). It is of interest, therefore, to assess the

relationship between the media specific half-life values

adopted under REACH, for example, and the default POV value

defined in the OECD Tool for a wide range of chemicals. To do

this a chemical space of POV, covering the region of

�6 < log air–water partition coefficient (KAW) < 3 and

0 < log octanol–air partition coefficient (KOA) < 12 on a grid

of 1 log unit, while holding the half-life values for air, water,

and soil constant at 2 d, 60 d, and 180 d, respectively,

consistent with a vP substance under REACH has been

calculated, representing 60 hypothetical chemicals. Note that

this approach assumes that the half-lives in soil and sediment

are equivalent.

In order to estimate the response time a substance may

have to emission reductions in the environment, the time for

the mass of each of the 60 hypothetical chemicals described

above with the same REACH vP half-life values, to be reduced

by 95%, based on results from the GloboPOP model has been

estimated. Specifically, chemical is emitted as a 1-year

steady-state pulse (i.e. assuming steady-state emissions for

1 year) using three different emission scenarios, whereby

chemical is emitted 100% to: (1) the lower air; (2) water; and (3)

soil compartments of each climatic zone in the GloboPOP

model based on the emission scenario described by Wania

(2003). In this scenario, chemical is emitted based on the

distribution of the human population, whereby the majority

of chemical is emitted to the northern hemisphere,

with 34.15% being emitted to the north-subtropic zone

(Wania, 2003).

3. Results and discussion

3.1. Persistence and LRTP metrics

Estimates of POV and LRTP based from output of the OECD Tool

are reported in Table 2. Generally, with the exception of those

identified as high production volume chemicals, substances

that have been listed for authorization as PBT substances

under REACH, as a POP under the Stockholm Convention, or as

having ozone depleting capability, have POV values that exceed

195 d. The POV values for substances identified as high

production volume chemicals (HPVCs), are <195 d, as is D5,

a substance of emerging concern.

The OECD Tool provides additional information regarding

the transfer efficiency of a substance. Generally, for sub-

stances described as PBT under REACH or as a POP under the

Stockholm Convention in Section 2 above, and included in

Table 2, high CTD values correspond with relatively large TE.

Compare, for instance, the results for HCB, which has a CTD

value of about 105 km and a TE of 282%. This is not necessarily

always the case, however, and there may be some substances

Table 1 – Physical chemical properties of chemicals of concern.

Chemical name Log KOW Log KAW Log KOA HLair (d) HLwater (d) HLsoil (d) HLsed (d)

REACH PBT candidate list

Anthracene 4.68a �3.0a 7.7a 0.4i 23j 230j 708j

Musk xylene 4.9b �4.6b 9.5b 19i 365k 2230k 2230k

HBCDD 5.63c �3.5c 9.13c 1.1i 15l 135l 135l

Chlorinated paraffins (short chain, C10–13) 5.06–8.12d �3.9 to �2.2d 8.2–9.8d 1.3i 2230m 2230m 708m

TBTO 4.05e �5.1e 9.15e 0.2i 70e 2230e 2230e

Stockholm Convention POPs

PCB-28 5.7a �1.9a 8.1a 23j 708j 2230j 2230j

a-HCH 3.98a �3.6a 7.37a 71j 230j 71j 2230j

HCB 5.5a �1.4a 7.2a 230j 2230j 2230j 2230j

Montreal Protocol substances that deplete the ozone layer

CFC-11 2.53f 0.6f 1.93f 29,713f 277f 554n 554n

CFC-12 2.16f 1.8f 0.38f 277f 62f 554n 554n

Carbon tetrachloride 2.83f 0.1f 2.74f 3646f 277f 554n 554n

U.S. high production volume chemicals

Cyclohexane 3.44f 0.9f 2.54f 1.1f 104f 230j 230j

Toluene 2.73f �0.6f 3.3f 1.4f 23f 230j 230j

Acetone �0.24f �2.8f 2.55f 37f 7f 23j 23j

Methanol �0.77f �3.7f 2.96f 7f 7f 23j 23j

Phenol 1.46f �4.9f 6.33f 0.3f 2.3f 7j 7j

Substances of emerging concern

a-Endosulfan 4.94g �3.55g 8.49g 2.4i 2794g 5583g 18,167g

D5 8.03h 3.13h 5.07h 10i 71h 2230h 2230h

a Beyer et al. (2002).b Tas et al. (1997).c Information reported in the EU risk assessment for HBCDD (technical mixture) (ECHA, 2010).d Feo et al. (2009).e Information in the EU risk assessment for bis(tributyltin)oxide (ECHA, 2010).f Muller-Herold et al. (2005).g Shen and Wania (2005).h Information in the EU risk assessment for D5 (ECHA, 2010).i Estimated from the AOPWIN programme.j Mackay et al. (2006).k Information in the EU risk assessment for musk xylene (ECHA, 2010).l Estimated from the BIOWIN programme.m Information reported in the EU risk assessment for chlorinated paraffins (ECHA, 2010).n Assumption that the soil half-life is double the water half-life, and assuming that sediment half-life is equivalent to the soil half-life.


that have relatively high LRTP but their ability to deposit to

remote environments may be limited. A good example of this

behaviour can be seen for D5, which has a CTD value of about

5000 km but limited TE (0.01%).

Interestingly, the TE for CFC-11 and carbon tetrachloride

are observed to be very high, 1250% and 800%, respectively.

These substances are also estimated as having very long

persistence and LRTP, largely as a consequence of their long

atmospheric half-lives and relatively high KAW, resulting in

these substances as being characterized as persistent ‘fliers’

(Wania, 2006). It is curious to note that the eACP10 reported in

Table 2 for CFC-11 and carbon tetrachloride is reported to be

very low, as compared to the other substances investigated,

this is in contrast to the TE values reported from the OECD

Tool. In other words, while the OECD Tool implies that these

substances would be effectively deposited, the GloboPOP

model suggests that the ACP of these substances is likely to

be negligible. It is thus apparent that there is a difference in

the parameterization between these two models that results

in a significant difference in assessing the environmental

fate and behaviour of substances characterized as persistent

fliers.

A possible explanation for the difference between the two

models likely relates to differences in the parameterization of

the atmospheric compartment. The OECD Tool models the

atmosphere as one box, with a height of 6 km and a surface

area equivalent to the surface of the Earth (Wegmann et al.,

2009). GloboPOP, on the other hand, includes four atmospheric

compartments in each of the 10 climatic zones included in the

model (Wania et al., 1999). In GloboPOP, for instance, advective

velocities between the atmospheric boundary layer (height

1.3 km) and the lower troposphere (height 5 km), favour

upward transfer for the northern hemisphere. In the OECD

Tool, the mass transfer coefficient between the troposphere to

stratosphere is 0.064 m/h, and is based on a 7.4-year half-life in

the troposphere (Wegmann et al., 2009). The difference

between the two models implies more efficient vertical

transfer of chemicals with characteristics of persistent fliers

away from the atmospheric boundary layer to the strato-

sphere and less efficient deposition, particularly to the Arctic,

Fig. 1 – Chemical space of overall environmental

persistence for chemicals with media specific half-lives

consistent with a REACH vP substance, i.e. air half-life = 2

d; water half-life = 60 d; soil and sediment half-life = 180 d.

Green area indicates chemicals with POV < 180 d; red area

indicates POV > 210 d.

Table 2 – Persistence and long-range transport metrics, defined as the overall environmental persistence (POV);characteristic travel distance (CTD); transfer efficiency (TE); and the Arctic contamination potential (ACP), where eACP10

describes the ACP after 10 years of global steady-state emissions consistent with calculations presented by Wania (2003).

Chemical name OECD Tool eACP10 (%)

POV (d) CTD (km) TE (%)

REACH PBT candidate list

Anthracene 310 196 0.021 0.03

Musk xylene 3034 2698 9.34 0.41

HBCDD 194 488 0.21 0.04

Chlorinated paraffins (short chain, C10–13) 3142 620 0.23 0.22

TBTO 2350 173 0.01 0.08

Stockholm Convention POPs

PCB-28 2050 10,900 10.1 0.62

a-HCH 317 6000 32.1 0.32

HCB 1860 99,712 282 1.85

Montreal Protocol substances that deplete the ozone layer

CFC-11 34,726 1.2 � 106 1250 8.2 � 10�4

CFC-12 400 1.2 � 105 9.2 2.7 � 10�7

Carbon tetrachloride 4850 7.5 � 105 800 2.7 � 10�3

U.S. high production volume chemicals

Cyclohexane 73 550 0.0002 4.9 � 10�10

Toluene 27 691 0.002 1.2 � 10�14

Acetone 26 6780 19.6 3.8 � 10�13

Methanol 19 2030 3.8 1.2 � 10�16

Phenol 9 157 0.04 2.3 � 10�48

Substances of emerging concern

a-Endosulfan 5278 1128 1.12 0.52

D5 92 4967 0.01 6.3 � 10�6


in GloboPOP. Whereas in the OCED Tool, the parameterization

of a single atmospheric compartment combined with a

relatively long troposphere half-life, results in more efficient

deposition and less efficient troposphere to stratosphere

mixing, relative to the GloboPOP model. Without conducting

a thorough sensitivity analysis, however, it is unclear which of

the two models better characterizes the environmental fate

and behaviour of the persistent fliers.

3.2. Relationship between REACH vP criteria and theOECD Tool

Generally, the consistency of results between substances

suspected to be PBT or POP and the POV threshold value used by

the OECD Tool suggests that the tool has the potential to

identify a chemical space, which relates the partitioning

properties of a substance to POV. The results are illustrated in

Fig. 1, whereby the red area to the right of the figure represents

chemicals that would have POV values > 210 d, and the green

area to the left, represents chemicals that would have

POV < 180 d. The area in orange illustrates 180 d < POV < 210

d, and the black hatched area represents the threshold value of

195 d. The estimated POV values are based on results for 60

hypothetical chemicals with partitioning properties covering

the chemical space shown in Fig. 1, and with media specific

half-lives consistent with a REACH vP substance, i.e. air half-

life = 2 d; water half-life = 60 d; soil and sediment half-

life = 180 d.

Fig. 2, on the other hand, provides an illustrative example

to demonstrate how the results for the actual chemicals listed

in Table 2 compared against the results shown in Fig. 1. This

has been done by superimposing the partitioning properties of

each of the chemicals listed in Table 1 onto the POV chemical

space (Fig. 1). From Fig. 2 it can be seen that substances that

have half-life values � REACH vP criteria with log KOA values

> 6, generally have POV > 195 d (Table 2). There are, however, a

few exceptions, which include the highly persistent CFCs

shown as red circles, which have log KOA < 6 and POV > 210 d

Fig. 2 – Location of individual chemicals, based on their

partitioning properties superimposed on the chemical

space of overall environmental persistence. Note that the

media specific half-lives for each of the substances shown

may be higher or lower than the REACH vP criteria used in

creating Fig. 1.

Fig. 3 – Chemical space of overall environmental

persistence for chemicals with media specific half-lives an

order of magnitude greater than a REACH vP substance, i.e.

air half-life = 20 d; water half-life = 600 d; soil and

sediment half-life = 1800 d.


(influenced by their exceptionally long half-lives in air, as

shown in Table 1), and HBCDD, which has a log KOA > 6 and

POV = 194 d (influenced by media specific half-lives that are <

REACH vP criteria used in producing Fig. 1). Caution is thus

recommended when interpreting the POV of a substance based

on where its partitioning properties place it in the POV space

shown in Fig. 2, since the actual value of POV will be strongly

influenced by the media specific half-life values. For instance,

the POV space shown in Fig. 1 is based on the vP half-life values

used under REACH (i.e. air half-life = 2 d; water half-life = 60 d;

soil and sediment half-life = 180 d), but individual chemicals

may have half-life values that are significantly more (such as

the CFCs and carbon tetrachloride listed in Table 1) or less

(such as HBCDD and the high production volume chemicals

listed in Table 1) than the REACH vP criteria. Nevertheless, the

good agreement for the remaining SVHC, suggests that the

REACH vP criteria combined with POV estimated by the OECD

Tool provides an effective metric for identifying potential

persistent substances.

A notable difference arises, however, for substances with

log KOA < 6, such as seen for D5, which is currently undergoing

PBT assessment under different jurisdictions, including under

REACH. For D5, media specific half-life values that have been

either measured or estimated for air, water, and sediment all

exceed the criteria for a vP substance (see for instance Table 1),

however, the POV is estimated to be less than the OECD Tool

threshold value of 195 d. This is due to the partitioning

behaviour of D5, with the bulk of the mass (i.e. 99.7% for the

emission to air scenario) being associated with the atmo-

sphere. Consequently, whereas the media specific half-life

values exceed the REACH cut-off criteria, the reactivity in air is

sufficiently fast to cause it to have an estimated POV of only 92

d (Table 2). This estimate, however, is strictly based on the

behaviour of D5 in the OECD Tool model environment, as well

as on the chemical properties defined in the EU risk

assessment (Brooke et al., 2009), which does not include an

uncertainty analysis of the input properties.

A sensitivity analysis was performed to assess how

changes in media specific half-lives might shift the region

in the chemical space to increase the region where POV exceeds

195 d. This was done by increasing each of the media specific

half-life values by a factor of 10, i.e. air, water and soil half-life

values of 20 d, 600 d, and 1800 d, respectively. The results are

shown in Fig. 3. The increase in media specific half-life values

causes substances identified as ‘swimmers’ (log KAW < �2, and

log KOA < 6) (Wania, 2006), and which now have a half-life

value that is significantly in excess of the vP REACH criteria, to

have an POV > 195 d. An additional shift in the POV > 195 d can

be observed to now include substances with log KOA > 5.

Generally, it can be seen that even with significantly higher

media specific half-life values (order of magnitude greater

than the regulatory cut-off values) that the POV of substances

to the upper left of the chemical space continue to have POV

values < 195 d, and that for this region of the chemical

space the POV is largely influenced by the air half-life, with

surface half-life values having minimal influence. Thus, for

substances identified as fliers, having log KOA < 5 and

log KAW > �2 (Wania, 2006), hazard assessments of persis-

tence, based on a media specific half-life approach, may show

disagreement with the POV approach, largely because of the

inconsistencies between the OECD Tool estimates of POV and

regulatory cut-off values.

3.3. Relationship between partitioning properties and theoccurrence of a lag

In an effort to address the issue of how environmental

response times to reduced emissions can provide guidance for

chemical assessment priority setting the relationship between

the potential for a substance to exhibit a lag between the time


when emissions stop and when 95% of the substance has been

removed from the global environment are investigated. The

potential to become a ‘legacy’ pollutant is investigated based

on the output of the GloboPOP model for each of the 60

hypothetical chemicals used in constructing Fig. 1. It is notable

that mode-of-entry is important, thus the lag-times using

three different emission scenarios; to air, water, and soil,

based on levels that remain in the environment following a 1-

year steady-state emission to each of the respective compart-

ments and climatic zones is assessed. The results are shown in

Fig. 4. It is important to note that the upper limit of the lag-time

is constrained by the vP REACH media specific half-lives that

are used in the model. Thus, for substances that partition

largely to sediment, and which have a vP sediment half-life

consistent with the REACH criteria of 180 d, a lag-time

corresponding to 5 half-lives, or the time for 95% of the

chemical to be removed from the environment, will occur at

about 2.5 years (Fig. 4). Clearly, chemicals with media specific

half-lives greater than the vP cut-off criteria will experience

lag-times in excess of 2.5 years, and will subsequently have a

higher likelihood of being defined as a ‘legacy’ substance. For

substances that fall into the region of concern identified in

Fig. 4, and for which data regarding their environmental fate

and behaviour are limited, early authorization assessment is

thus warranted.

The area shown in green represents chemicals that have

media specific half-life values consistent with the vP criteria,

but have lag-times < 1 year. Substances that fall within this

area of the chemical space will likely respond quickly to

Fig. 4 – Proposed chemical space defining the relationship betwe

precautionary decision-making process. Areas in green indicate

areas in red indicate high potential for the substance to be a le

regarding the legacy potential of the substance.

emissions reductions, and thus delaying the implementation of

regulatory measures will likely result in the substance not

having a legacy associated with its use. For these substances it

is recommended that postponing actions in favour of acquiring

more data will only help to strengthen the regulatory process.

The area shown in orange represents substances that may

have 1 year < lag-times < 2.5 years, and that have half-life

values consistent with the vP REACH criteria. Assessing the

response times of these substances is likely to be less certain

and more problematic, and will depend largely on the extent to

which the media specific half-life exceeds the REACH criteria,

as well as on mode of emission. Rapid processing under

REACH authorization, for instance, may or may not be

warranted, depending on the quality of data initially available

to assess degradation and mode-of-entry. Consequently,

prioritizing actions for these substances will be of higher

concern than for substances falling in the green area of the

space in Fig. 3, but ranking the prioritization of actions for

substances that fall within the orange area of the space will

likely need to be based on the properties of individual

chemicals, i.e. on a case by case basis.

The results shown for the different emission scenarios in

Fig. 4 suggest that of the three emission scenarios, those

associated with emissions to air and soil are generally

consistent between each other. For instance, chemicals with

lag-times in excess of 2.5 years in both emission scenarios are

found in the lower right of the figure, generally having

log KOA > 8 and log KAW < 1. It is notable that the majority of

identified PBT substances, including the PBT candidate

en the physical–chemical properties of a substance and the

low potential for the substance to be a legacy contaminant;

gacy contaminant; areas in orange indicate uncertainty


substance, endosulfan, fall into this region. The largest

difference between the air and soil emission scenarios regards

the area of uncertainty (shown in orange). This region

captures more of the fliers in the emissions to soil scenario,

which is an artifact of the partitioning properties of these

volatile and hydrophobic substances. For these substances,

the fraction remaining in soil, as a result of direct emissions to

soil, will be greater than the fraction in soil when emitted

directly to air. Consequently, the lag-times will be greater

when emitted to soil than to air because of the longer half-life

in the soil compartment. For the air emission scenario,

however, the region of uncertainty is shown to capture more

of the swimmers. This is because a larger fraction of the

swimmers that are emitted to the atmosphere will eventually

be deposited to the oceans, where the longer half-life in

marine water will influence the magnitude of the lag.

The results shown in Fig. 4 for the water emission scenario

indicate a significant region of uncertainty for identifying

potential ‘legacy’ or persistent substances, with no regions of

the space identifying chemicals having a lag in excess of 2.5

years. The reason for this is that the emission to water

scenario results in losses from the environment to be largely

influenced by the water half-life, which is defined as 60 d.

Consequently, the time for 95% removal from the environ-

ment will be 300 d. Thus, it is unlikely, given these

assumptions that this emission scenario will result in lag-

times > 2.5 years in the GloboPOP model environment. In this

instance, obtaining good quality data, particularly regarding

the water half-life, in order to effectively assess the urgency

for regulatory authorization is strongly recommended. It is of

interest to note that the majority of substances identified as

legacy POPs under the Stockholm Convention have emission

scenarios consistent with emissions that have largely been

associated with air and soil, with minimal amounts going

directly to water (Breivik et al., 2006). For household consumer

products and pharmaceuticals, however, down-the-drain

scenarios are more likely, and therefore emissions to water

will dominate their behaviour. For these substances removal

via waste water treatment processes would need to be

considered, since treatment of waste water prior to discharge

into the natural environment may result in negligible amounts

being released, further complicating the assessment of

‘legacy’ potential for chemicals largely emitted to water.

A curious observation in Fig. 4 is that a lag-time is unlikely,

regardless of emission scenario, for chemicals with

�2 < log KAW < 0 and 3 < log KOA < 4. For these substances

delaying actions to obtain empirical data to support regulatory

measures would always benefit the PBT assessment, regard-

less of mode-of-entry. The assumption, however, is that

toxicity and bioaccumulation data are uncertain. Obviously, if

the chemical is known to be highly toxic or bioaccumulative,

delaying actions would not be supported for chemicals in this

region of the chemical partitioning space.

3.4. Regulatory implications and recommendations

Currently, two substances listed in Table 1 are undergoing PBT

or POP assessment; D5 and endosulfan. Whereas the analysis

described above implies that endosulfan has properties that

make it a likely candidate for persistence, observations of the

model outputfor D5 is lessconclusive.D5 hasbeenidentifiedasa

substance having media specific half-life values that exceed the

vP REACH criteria for persistence, and is characterized as a flier,

having an POV < 195 d (based on the OECD Tool persistence

thresholdvalue), a lag-time<1year, when emitted toair,and a 1

year < lag-time < 2.5 years when emitted to water or soil,

resulting in inconsistenciesbetween the various hazard metrics

proposed. It has also been shown that D5 has significant LRTP,

but limited potential to contaminate remote environments, as

demonstrated by various metrics, including TE and ACP.

According to the EU risk assessment for D5 (Brooke et al.,

2009), the chemical is largely emitted to the atmosphere (90%),

with 10% being emitted to waste water, of which, 95% is

assumed to be removed in the waste water treatment plant,

resulting in 0.5% of the substance being emitted directly to

water. This implies that assessing the lag-time of D5 will depend

on where thepropertiesof the chemicalplace it in the emissions

to air scenario of Fig. 4. Based on measured physical–chemical

properties used in the EU risk assessment (Brooke et al., 2009)

the environmental lag-time of D5 is likely to be minimal (<1

year), and consequently allowing time to acquire the necessary

data to support the regulatory process will only strengthen any

regulatory decisions regarding the future use of the substance.

Indeed an outcome of the preliminary EU risk assessment

(Brooke et al., 2009) is a provision that allows industry time to

voluntarily undertake a testing programme to address impor-

tant data-gaps identified in the risk assessment. A notable

limitation of this approach, however, is that it does not

quantitatively account for the uncertainty in the physical–

chemical property data for D5, thus the uncertainty in measured

data may cause it to fall elsewhere in the partitioning chemical

space shown in Fig. 4.

Assuming reliable partitioning data are available, however,

it is believed that the approach described in this study can be

useful for prioritizing regulatory actions. For instance, identify-

ing when it is appropriate to provide sufficient time to obtain

data needed to strengthen the regulatory process (i.e. refine the

risk assessment), and when postponing actions might prove

unwise. It is thus recommended that regulatory bodies consider

the urgency to act in relation to the physical–chemical

properties of the chemical under PBT or POP assessment, and

that the approach presented in this study can provide guidance

for priority setting for persistence assessments.

Acknowledgements

The study has greatly benefited from comments from Michael

McLachlan, Oliver Price, Ian Malcomber, Stuart Marshall, and

Roger van Egmond, as well as comments from two anonymous

reviewers.

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Todd Gouin is an environmental fate modeller, recently joiningthe Safety & Environmental Assurance Centre at Unilever, basedin the UK, June 2009. His research interests focus primarily on theenvironmental fate and transport of persistent, bioaccumulative,and toxic substances.

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The precautionary principle and environmental persistence: prioritizing the decision-making process