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Medical Surveillance for

Silica-exposed Employees

Kenneth Corbet MD FRCPC

Consultant, Occupational Medicine

Clinical Associate Professor

University of Calgary

September 29, 2014

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No conflicts to declare

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Learning Objectives:

Describe the objectives of a medical surveillance

program for silica-related disease

Interpret regulations administrative vs. risk-based

Who is silica-exposed and can we estimate risk?

Describe the essential elements of screening

Determine when medical referral is indicated.

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Definitions & Principles

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Medical Surveillance

Medical surveillance is periodic screening for defined

clinical criteria in groups of employees who are:

exposed to a workplace hazard

at risk of an occupational disease

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At risk

cohortCase Referral

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Objectives of the Surveillance Program:

Early recognition and diagnosis of chronic silicosis

Limit further exposure, limit disease progression

and respiratory impairment, limit cancer risk

Meet case reporting duties (e.g. OHS, WCB, IIHL)

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Biological

Onset of

Disease

Disease

Detectable

by Screening

Clinical

Presentation

Of Disease

Impairment

Disability

Death

Onset of

Exposure

The natural history of a disease

A: How long after the onset of exposure is the disease

detectable by screening?

B: How much lead time does screening offer before the

person would present clinically?

C: Does early intervention lead to an improved outcome?

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A B C

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Medical Surveillance Criteria

Medical surveillance may be warranted if:

achieving the OEL relies heavily on employee behaviors (e.g.

administrative rules, PPE)

the true exposure of the employee poses a health risk

a medical test or examination can identify cases of possible

disease (and meets usual screening criteria)

early intervention can prevent or limit the progression

of an occupational disease.

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Employee Perceptions

If Im eligible for monitoring, I must be at high risk for lung disease.

If I participate in monitoring, I wont get lung disease.

Youre mean youre not screening for lung cancer?

= Effective, ongoing, consistent communication:

accurately describe the health risks

and benefits of participation.

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Silica & Silica-related Disease

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Silica-related Diseases

The target disease for medical surveillance is

Chronic (simple) silicosis

Airways obstruction or restriction of lung capacity

The target for medical surveillance is NOT

Lung Cancer

Autoimmune or renal disease

Accelerated silicosis or progressive massive fibrosis

Acute silicosis12

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Alberta Regulations

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Restricted area means an area of a work site where there is a

reasonable chance that the concentration of silica exceeds the

occupational exposure limit (OEL)

Any worker who may reasonably be expected to work

in a restricted area for at least 30 work days per year requires

medical surveillance every two years.

Alberta Occupational Health and Safety Code (2009)

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How do you provide administrative criteria for medical

surveillance that are applicable to smaller businesses, with

changing workforces, work locations, and work processes

but also provide exposure-based criteria for larger businesses

with more stable workforces and well-characterized exposures?

and can both types of criteria be based on a valid assessment

of risk?

The Challenge for Regulators

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Occupational Exposure Limit (OEL)

1. the target (or allowable) daily exposure to

a hazard achieved through exposure controls (including

personal protective equipment)

used by occupational hygienists

2. a level of continuous, daily, occupational exposure that

should not lead to serious adverse effects over a working

lifetime

more relevant for medical surveillance

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Exposure & Health Risk

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Winter Spring Summer Fall

Alberta OEL = 0.025 mg/m3

Full

shift

respirable

sili

ca (

mg/m

3)

Full-shift levels exceed the OEL:

implement additional exposure controls;

may require medical surveillance

Annual dose remains less than the annual OEL

One exposure scenario Alberta Oilsands

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On the other hand

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Similarly Exposed Groups

A group of employees who share similar tasks, tools, equipment,

work locations, and schedules and are presumed

to have similar ranges of exposure to a hazard.

At least 6-10 full-shift samples (ideally 20-30) are required

per SEG to characterize exposure distribution

through statistical methods

Sampling must include all phases of the production cycle.

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Num

ber

of

full-

shift sam

ple

s

Full-shift exposure (ug/m3)

Detection limit

1 2

OEL What is the exceedance fraction

(shifts above the OEL) ?

1. Small fraction of samples below detection limit = Parametric methods

2. Large fraction of sample below detection limit = Bayesian methods

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What is the Geometric Mean Dose

or the GMD 95%UCL?

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Adapted from: Occupational Exposure to Respirable Crystalline Silica

Review of Health Effects Literature and Preliminary Quantitative Risk Assessment.

US Occupational Safety and Health Administration (2010)

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Adapted from: Occupational Exposure to Respirable Crystalline Silica

Review of Health Effects Literature and Preliminary Quantitative Risk Assessment.

US Occupational Safety and Health Administration (2010)

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Program Components

& Process

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Risk Communication an explanation of the health effects

of silica, current exposure levels, estimated risks, and the

elements and benefits of a medical surveillance program

Periodic Reassessment - this should vary depending on the

cumulative exposure assessment for the individual

Voluntary - written refusal to undergo part or all of a health

assessment

Risk Communication & Consent

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Baseline Questionnaire

Dust Exposure History

Exposures to asbestos, silica, coal, or other industrial dusts

and carcinogens - occupational and non-occupational (smoking)

= risk for airways obstruction

Silica Exposure History

History of work in jobs with silica exposure; correlate to

published studies or job exposure matrices (JEM) to estimate

past exposure to silica in mg/m3-years

= risk for radiographic changes27

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Baseline Questionnaire

Medical History

Cough, wheeze, dyspnea, sputum (i.e. standardized

respiratory symptoms questionnaire)

Past diagnoses relevant to the interpretation

of the chest radiograph or spirometry

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Digital imaging

Certified technician

Standard technique

Interpretation by a radiologist

Archival data files

May require ILO interpretation

Chest Radiograph

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Case Definition:

work with a radiologist

small diffuse nodular

opacities OR eggshell

calcification

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Chest Radiograph

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At autopsy, pathology findings correlate poorly with

a recent chest radiograph (2-3 years):

Pathology % with rounded opacities (ILO 1/1)

Slight silicosis 25%

Moderate silicosis 46%

Marked silicosis 74%

REFERENCE:

Hnizdo et al (1993) as cited in ACGIH Documentation of the TLV (2010)

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Certified testing equipment

Certified testing staff

Calibration

Technique and quality of testing *

Interpretation and recommendations(ACOEM 2011 Guidance Statement)

Longitudinal analysis of FVC and FEV1(Hnizdo 2010; Spirola software (CDC)

Spirometry

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Case Definition = < LLN for FVC or FEV1

OR

> Normal longitudinal decline

in capacity or flow

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Can we do a better job?

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Biological

Onset of

Disease

Disease

Detectable

by Screening

Clinical

Presentation

Of Disease

Impairment

Disability

Death

Onset of

Exposure

The natural history of a disease

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Spirometry

Chest radiographHigh Resolution CT?

Biomarkers of

Inflammation

(exhaled nitrous

oxide)

DLco?

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High Resolution Computed Tomography (HRCT)

HRCT is superior to chest radiograph for:

the diagnosis of silicosis

the early detection of air trapping and emphysema

the monitoring of progressive silicosis and PMF

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Radiation Dose

High Resolution CT = 0.98 mSv

standard CT scan = 6.5 mSv

conventional chest examination = 0.085 mSv

Background annual radiation dose = 2-3 mSv

Can exposure be reduced through a modified

silica protocol for HRCT?36

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Thank you!

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