Introduction to exposure standards (ADIs, TDIs, RfDs, OELs and DNELs)

Gunnar Damgård Nielsen, PhD, Dr Sc (Pharm)

National Research Centre for the Working Environment, Lersø Parkallé 105, 2100 København Ø

E-mail: GDN@nrcwe.dk

Updated 29.07.2014

Purpose of the lecture• Do we need exposure standards?• Risk assessment approach (threshold and

non-threshold mode-of-action (MOA))• No observed adverse effect level (NOAEL)

and UFs/SFs/AFs, and risk assessment• Mixtures• Risk assessment for carcinogens (EU

SCOEL as an example)• Shortcuts for “OELs” for drugs• Exposure of no toxicological concern

Coal workers´ pneumoconiosis (CWP)Exposure limits/standards/guidelines for health protection

Coal workers’ pneumoconiosisMinimata Bay - CH3-Hg+ effects due to environmental Hg pollution

Risikovurdering - Risk Assessment

A. Farevurdering - Hazard Identification(iboende egenskaber - inborn properties)

B. Dosis-effekt/respons sammenhæng - Dose-effect/response relationship

(fx NOAEL, LOAEL or BMD for critical effect(s))

C. Eksponeringsvurdering - Exposure Assessment)(concentration/dose)

Risikokarakterisering - Risk Characterisation(incidence/severity/likelihood)

Risk assessment of exposures follows the general approaches for risk assessment from the 1980’s

Exposure-response - the classical approach

Two possibilities

NOEL

Linear non-threshold model

Threshold model

No observed adverse effect level (NOAEL)

The classical approachSubstances with NOAEL:

1. Standards/guidelines: ADI/TDI/RfD/OEL/DNEL= NOAEL/(Π fi)- fi=uncertainty factor (UF)/safety factor (SF)/assessment factor (SF)*- used in risk assessment

2. No standard/guideline: MOE/MOS=NOAEL/Exposure

Substances without NOAEL (threshold):

Unit risk (WHO: lifetime risk with a lifetimeexposure to 1 µg/m3) for carcinogenic compounds

*Extrapolation factor (f): NOAEL=LOAEL/f, where f may be from 2 to 10During a day (24 h), an adult subject inhales 20 m3

During an 8-hour workday, it is assumed that a 70 kg man inhales10 m3

DefinitionsAdaptive response: A cell or organism will survive in the new environment that contains the xenobiotic without impairment of function.Compensatory effect: Enable the organism to maintain overall function without further enhancement or significant cost (~NOAEL).Adverse effects: Change in biochemical functions, functional impairments, or pathologic lesions that impair the performance of organism, and/or reduce its ability to compensate for additional stress, or an increase in susceptibility to other influences.Critical effect: First adverse effect, or its known precursor, that occurs to the most (relevant or) sensitive species as the dose rate of an agent increase (~LOAEL). The highest NOAEL below this LOAEL is generally used in the dose-response , and the focus is on determining this NOAEL in a sensitive population

Dourson et L. Advancing human health risk assessment: integrating recent advisory Committee recommendations. Crit Rev Toxicol 2013; 43: 467-492.

Hazard identification(NOAEL and /or LOAEL)

Exposure assessment

Standard setting(e.g. ADI, TDI, RfD, OEL, DNEL)

Risk characterisation (comparison of standard with exposure)

AcceptableUnacceptable(risk management)

Model 1: Substances with NOAEL and with standardsAn authoritative risk assessments can be performed

The Interdepartmental Group on Health Risks from Chemicals. Uncertainty factors:their use in human health risk assessment by UK Government. MRC Institute for

Environment and Health. University of Leichester, United Kingdom.

UFs: default or mechanism based

1 ≤ EXP/Standard 1 > EXP/Standard

Model 2: Substances with NOAELs and where no standard is set

Hazard identification(NOAEL and/or LOAEL)

Exposure assessment (EXP)

Risk characterisation: direct comparison of of NOAEL with exposure – e.g. NOAEL/EXP (margin-of-exposure (MOE), margin-of-safety (MOS))

Risk management decision-making

Acceptable Unacceptable: risk mangement

The Interdepartmental Group on Health Risks from Chemicals. Uncertainty factors: their use in human health risk assessment by UK Government. MRC Institute for Environment and Health. University of Leichester, United Kingdom.

International Programme on Chemical safety (UNEP+ILO programme) default values (2005)

Inter-species differences (combined AF=10)• Toxicokinetics: AF=4.0• Toxicodynamics: AF=2.5

Intra-species differences (combined AF=10)• Toxicokinetics: AF=3.2• Toxicodynamics: AF=3.2

Composite AF=(Inter-species AF) x (intra-species AF) = 10 x 10 = 100

Replace default values with (mode-of-action) chemical specific AFs

Dourson et L. Advancing human health risk assessment: integrating recent advisory Committee recommendations. Crit Rev Toxicol 2013; 43: 467-492.

Food additives US EPA regulations OELs and REACH Acceptable daily intake (ADI) (contamin.: Tolerable daily intake (TDI))

Reference dose (RfD) OEL (e.g. SCOEL) and DNEL (REACH)

ADI/TDI=NOAEL/100

AF animals to humans: 10AF within humans: 10Animals/positive lists

Assumed to include mixtures for compounds with different MOA

RfD=NOAEL/composite AFRfD=BMD/composite AF

AFs for:Extrapolation, dosimetricadjustment, length of study, clinical severity, insufficient data.Composite AF: < 100 to > 100

OEL=NOAEL/composite AFComposite AF often from2-10 and often from human inhalation studies; SCOEL

REACH DNELs: Interspecies AFs (allometricscaling; toxico. Dyn. 2.5), workers (5), subacute to subchronic (3), subchronicto chronic (2)

Dourson et al. Crit Rev Toxicol 2013; 43: 467-492. Magnuson et al. Food Addit Contam Part A 2013; 30: 1147-1230.Nielsen GD, Øvrebø S. Regul Toxicol Pharmacol 2008; 51: 253-269. ECHA. Guidance on information requirements and chemical safety assessment. Chapter R.8. Characterisation of dose [concentration]-response for human health. May 2008. http://guidance.echa.europa.eu.

Occupational Exposure Limits (OELs) – types of standard

• Health based OELs do not take into account economic and technical feasibility (e.g. SCOEL, MAK, ACGIH)

• Administrative norms and legally bindingOELs, which have taken economic and technical feasibility into account

The health based (“indicative”) OELs set by the EU Scientific Committee on Occupational Exposure Limits (SCOEL)

Health (NOAEL) based OELs:• Exposure 8 hrs/day, 5 days/week 240 days/year for up to 45 years• Time weight average exposure limits (TWA): 8-hour mean value• Short term exposure limits (STELs): Usually 15 min (irritants and

anaesthetics)• Health-based biological limit values (BLVs)

To search for SCOEL’s methodology for setting OELs and for documentations for evaluated compounds, consult:http://ec.europa.eu/social/main.jsp?catld=148&langld=en&intPageId=684

Notations:• Skin notation where absorption through the skin contributes significantly to the

body burden• Notation for respiratory and skin sensitizers, and for noise• Type of carcinogen (A, B, C, D); no health based OEL for A and B

Units for gases or vapours:• mg/m3 of air•ppm = ml (cm3)/m3 of air• conversion: 1 ppm = [M/24.45*] mg/m3 at 25ºC and 1 atm.

* Conversion to other temperatures at 1 atm: X =22.4 x (273+t)/273 ”t” in °C, M=molecular weight.

Units for aerosols:• mg/m3 of air

Units for fibres:• fibres/cm3 of air

OELs protect against airborne concentrations where no skin absorption occur

Exposures are nearly always to mixtures

Evaluation of mixtures with components with NOAEL• Direct assessment of dose-response of the mixture (e.g. tobacco smoke)

• Assessment of dose-response for an appropriate model mixture

• Independent effects (dissimilar MOA); acceptable if hazard quotients (HQi=Exposurei/Standardi) of each compound is < 1.

• Dose addition: Hazard Index (HI)=Sum (Exposurei/Standardi). Acceptable if the sum < 1. If HI>1, refine and calculate HIs for compounds with similar MOA. If each HI for refined calculation is < 1, the exposure is acceptable

• Toxicity Equivalent Factors, e.g. for PAHs, where BaP is set to 1 and the BaP equivalents are calculated and assumed to be additive

Dourson et L. Advancing human health risk assessment: integrating recent advisory committee recommendations. Crit Rev Toxicol 2013; 43: 467-492.

Evaluation of carcinogenic effects of chemicals

1) Hazard evaluation: Classification and labelling YES/NO

2) Risk assessment – recent trends based on experiences from OEL settings

Stages in the development of cancer

Valko M, Rhodes CJ, Moncol J, Izakovic M, Mazur M. Free radicals, metals and antioxidants in oxidative stress-induced cancer. Chem Biol Interact 2006; 160: 1-40.

World Health Organization. International Agency for Researchon Cancer. IARC Monographs onthe Evaluation of Carcinogenic Risksto Humans. Lyon.

Summary reports see:www.inchem.org ⇒ InternationalAgency for Research on Cancer (IARC)- Summary & Evaluations⇒Search

Hazard identification: Yes/no

http://monographs.iarc.fr/ENG/Classification/index.php

„Dose-Effect Relations in the Low Dose Range and Risk Evaluation“(Streffer et al. 2004)

Chemical carcinogen,causing tumours in humans and/or experimental animals

Non-genotoxicGenotoxic

DNA reactive,causing mutations

Genotoxicity only on chromosomelevel (e.g. spindle, topoisomerase)

Weak genotoxin,secondary mecha-nisms important

Borderlinecases

ClearlyDNA-reactive& initiating

C: Practical/apparentthreshold likely

D: Perfect/statisticalthreshold likely

A: No threshold,LNT model to apply

B: Situation not clear→ precautionary princ.→ LNT as default

Numerical risk assessment,technical feasibilities→ ALARA principle

NOAEL→ health-based exposure limits

Case Studies (Streffer et al. 2004)

=vinyl chloride (VC), =4-aminobisphenyl (4-ABP), =diethylnitrosamine (DEN),

No threshold, LNT model to apply:A

LNT as default assumption:= acrylonitrile, = acrylamide, = arsenic

B

Practical/apparent threshold:= formaldehyde, = vinyl acetate,

C

D Perfect/statistical threshold:=TCDD and other tumour promotors, = spindle poisons,=topoisomerase II poisons, = hormones

Distinction between B and C is most important !

Linear extrapolation• A default policy choice, intended to be health-

protective in the face of uncertainties (e.g. SCOEL group A and B)

• Stochastic assumption (proportional with exposure); a single heritable change to DNA can induce malignant transformation, causing cancer (not sufficient)

• Non-linearity or threshold can be due to uptake, transport, metabolism, excretion, receptor binding, DNA repair etc.

Dourson et L. Advancing human health risk assessment: integrating recent advisory Committee recommendations. Crit Rev Toxicol 2013; 43: 467-492

Carcinogenic substances with “practical” threshold

Vinyl acetate

Vinyl acetate: basis for risk assessmentCH2=CH-O-C(O)-CH3

CH3COO- + H+ CH3-CHO

Carboxylesterase

H+

DNA-protein cross-linking at high doses

Clastogenic effectCH3COO- + H+

Aldehyde dehydrogenase

Celle proliferation (mitogenic effect) and cytotoxic at low doses

Bogdaffy MS, Valentine R. Differenting between local cytotoxicity, mitogenesis, and genotoxicity inCarcinigen risk assessment: the case of vinyl acetate. Toxicol Lett 2003; 140-141: 83-98.

↓ pH[i]

Formaldehyde: a threshold carcinogen

Exposure (ppm) Number of animals at risk(A)

Number of animals with tumours(B)

B/A x 100

0.0 122 0 0

0.17 27 0 0

2.0 126 0 0

6.0 113 3 2.7

10 34 22 65

15 182 157 86

Results from three 2-year studies in rats, exposed for 6h/day, 5 days/week. (Schlosser et al. Risk Analysis 2003, 23, 473-487)

Key points: 1) repair of DNA adduct before fixed into mutation and 2) cell proliferation

Poorly soluble low-toxicity particles (PSP)

Lung deposition; depends on agglomerationNeutralisation of lung surfactants (LS)

No overloading of alveolarmacrophages (AM):Rats: T1/2 ~75 days for particles at < 6% of AM volume occupied

NOAEL(German MAK)

Overloading due to cumulative dose:Rat: T1/2 >>75 days for particles at >6% of AM volume occupied

Effects are due to the cumulative dose:• Chronic inflammation• Decreased clearance of

particles

Dysfunction of air-blood barrier that depends on the surface area and particle species – acute effect :Rat: ~75 µg LS/g lung and T1/2~ 0.5 day for LS

Acute lung (PMN) inflammation

Pauluhn J. The metrics of MWCNT-induced pulmonary inflammation are dependent on the selected testing regim. Regul Toxicol Pharmacol 2014; 68: 343-352.

Fiber toxicology: Dose - Dimension (L>5 µm, L/D≥3) - Durability

Fiber type Diameter (µm) a) Biological T1/2 b) Lifetime risk a,c)

Serpentine asbestos● Crysotile

~0.03 (fibril) ~ 8 years (L> 10µm); humans b)

Lung cancer risk (the same for serpentine and amphibole asbestos): 4 x 10-3 ~ 0.22 f/mlMesothelioma: 4 x 10-3 ~ 2.8 f/ml

Amphibole asbestos

● Crosidolite● Amosite

~0.06 (fibril) ~7 – 8 years; humansb)

~20 years; humans b)

Lung cancer risk: see aboveMesothelioma: 4 x 10-3 ~ 0.068 f/ml

Ceramic fiber 0.4 - 4 ~6 months; animals SCOEL assumes NOAELGlass fiber 1 - 5 ~10 days; animals SCOEL assumes NOAELMineral wool 4 - 7 ~10 days; animals SCOEL assumes NOAEL

a) Health Council of the Netherlands. Asbestos. Risks of environmental and occupational exposure. The Hage, 2010. Asbestosis is not a key effect as it is observed at ≥ 10 f/ml x year.

b) Agence francaise de sécurité sanitaire de l’environnemen (AFSSET). Expert appraisal for establishing occupational exposure limit values for chemicals. Asbestos fibres: assessment of the health effects and methods used for measuring exposire levels in the workplace. April 2009

c) Exposure for 8h/d, 5d/w for 40 years. Exposure-response analysis (linear): RR=1-K x f x d; K is a “constant”, including time of exposure and latency, and f x d is the cumulative dose.

Tubulin-binding molecules: Vinca alkaloids, Taxol and colchicine. Vinblastin – an example of a compound

expected to have a NOAEL

Indirect Genotoxicity Mechanisms and PossibleCriteria for Threshold Effects

§ Induction of aneuploidy§ Topoisomerase II poisons§ Oxidative stress§ Inhibition of DNA synthesis§ Steep dose-effect curve,

cytotoxicity involved

§ Endogenous compounds (?)§ Clastogens (?)

Kirsch-Volders et. al: Mutation Res. 464:3-11, 2000Madle et. al: Mutation Res. 464:117-121, 2000Pratt & Baron: Toxicol. Lett. 140/141: 53-62, 2003

„The dose-response relationship for a number of such

agents is generally accepted to show a threshold.

Sister chromatides decondense and nuclear envelopes appears

Sister chromatides are pulled towards opposite poles by microtubules

Chromosomes are aligned at the center of the cell and Properly attached to microtubules

The nuclear envelope breaks down and microtubulesare emanating

Centrome duplication and DNA Condenses into chromosomes

Centrosome dublication

Malmanche et al. The spindle assembly checkpoint: preventing chromosome mis-segregation duringMitoosis and meiosis. FEBS Lett 2006; 580: 2888-2895.

The red background indicates the level of cyclin B

Spindle assembly checkpoint (SAC)

Malmanche et al. The spindle assembly checkpoint: preventing chromosome mis-segregation duringmitosis and meiosis. FEBS Lett 2006; 580: 2888-2895

Vinblastin – a Vinca alkaloid that interacts with tubulin, the unit of microtubuli

Tubulin1 Tubulin2

Tubulin dimer

Vinblastin

Gigant et al. Structural basis for the regulation of tubulin by vinblastine. Nature 2005; 435: 519-522

Interaction of vinblastine with tubulin

Gigant et al. Structural basis for the regulation of tubulin by vinblastine. Nature 2005; 435: 519-522

Threshold of toxicological concern (TTC) for drugs

TTC:

• Carcinogens: 1 µg/day

• Potent or highly toxic drugs: : 10 µg/day

• Not likely to be potent: 100 µg/day

• Exceptions – does not apply to: Steroids, dioxines, aflatoxin-like compounds, azoxy- and N-nitroso-compounds

Dolan et al. Application of the threshold of toxicological concern concept to pharmaceutical manufacturing operations. Regul Toxicol Pharmacol 2005; 43: 1-9

Current and new risk assessment strategiesCurrent Systems toxicology: new type of studies and

in silico mathematical models

Exposure (exposome) including biomarkers

Exposure (exposome) including biomarkers

Molecular interaction• Metabolism (ADME)• Binding to macromolecules

Molecular interaction• Gene expression profiling (transcription)• Protein levels (proteomics) • Metabolites (metabolomics)• Lipid profiling (lipidomics)• High-content screening (broad panel of biomarkers)Animal and human studies

Biochemistry, pathology and histopathology

NOAEL/LOAEL

Data and information management (open-source software)

Computational platforms (structural descriptors,mechanisms, biological network and dose-response

Risk assessment

Sturla et al. Systems toxicology: from basic research to risk assessment. Chem Res Toxicol 2014; 27: 314-329.

Key referencesDourson M, Becker RA, Haber LT, Pottenger LH, Bredfeldt T, Fenner-Crisp PA. Advancing human health risk assessment: integrating recent advisory committee recommendations. Crit Rev Toxicol 2013; 43: 467-492.

European Chemical Agency (ECHA). Guidance on information requirements and chemical safety assessment. Chapter R.8. Characterisation of dose [concentration]-response for human health. May 2008. http://guidance.echa.europa.eu.

Magnuson B, Munro I, Abbot P, Baldwin N, Lopez-Garcia R, Ly K, McGirr L, Roberts A, Socolovsky S. Review of the regualtion and safety assessment of food substances in various countries and jurisdictions. Food Addit Contam Part A. 2013; 30: 1147-1220.

Nielsen GD, Øvrebø S. Background, approaches and recent trends for setting health-based occupational exposure limits: a minireview. Regul Toxicol Pharmacol 2008; 51: 253-269.

Olwenn VN, Scholze M, Kortenkamp A. Dispelling urban myths about default uncertainty factors in chemical risk assessment – sufficient protection against mixture effects? Environ Health 2013; 12: 53