Question 5 of 7: Skin Absorption Terminology, Canister Air-Purifying Chemistry, and Respiratory Protection Equipment

Question 5: Skin Absorption Terminology, Canister Air-Purifying Chemistry, and Respiratory Protection Equipment (20 marks: 8/6/6)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Skin-Absorption-Related Terminology

• (a) Corrosives — substances (typically strong acids or strong bases) that chemically destroy tissue on direct contact; a corrosive causes damage by direct chemical reaction with skin protein and lipid, not by an immune or allergic mechanism, and severity depends on concentration, pH extremity, and contact time.
• (b) Dermatitis — inflammation of the skin from chemical exposure, occurring either as irritant contact dermatitis (direct barrier damage, no prior sensitization needed) or allergic contact dermatitis (an immune-mediated reaction requiring prior sensitizing exposure); it is the most common occupational skin condition and can result from either single strong exposure or repeated mild exposure.
• (c) Eye damage — chemical splash injury to the eye, ranging from transient irritation to severe corneal damage or blindness depending on the agent's pH and contact duration; eyes are especially vulnerable because the cornea and conjunctiva are thin, permeable, and richly vascularized, with no protective epidermal layer analogous to skin.
• (d) Gastrointestinal absorption — chemical entry via the digestive tract, either from inadvertent ingestion (contaminated hands transferring material to the mouth, contaminated food/water/equipment in the work area) or via mucociliary clearance of inhaled particulate that is swallowed rather than exhaled; it is a route of entry distinct from, and often overlooked relative to, inhalation and dermal absorption.

(ii) Chemicals Used as Air Purifiers in Canisters

Air-purifying respirator canisters use a sorbent or catalytic medium matched to the specific target contaminant — a canister is never universal, and selecting the wrong chemistry provides no protection even though the respirator appears to function normally:

• Activated carbon/charcoal — adsorbs organic vapours (solvents) by physical adsorption onto its very large internal surface area.
• Impregnated activated carbon — carbon treated with a specific reactive impregnant to extend protection to gases poorly adsorbed by carbon alone (e.g. ammonia, mercury vapour, certain acid gases).
• Soda lime or a sodium/potassium hydroxide mixture — chemically neutralizes acid gases (CO₂, SO₂, HCl) by acid–base reaction rather than physical adsorption.
• Hopcalite — a manganese dioxide/copper oxide catalyst that oxidizes carbon monoxide to carbon dioxide, used specifically for CO protection where simple adsorption would not work.
• Particulate filter media — mechanical filtration of aerosols, dusts, and fumes; this removes particles, not gases or vapours, and is often combined with a chemical cartridge for combination hazards.
• End-of-service-life indicator media — some canisters incorporate a colour-change indicator that signals sorbent exhaustion, since sorbent capacity is finite and silent breakthrough of the target gas is otherwise undetectable to the wearer.

(iii) Respiratory Protection Equipment

• (a) Self-contained air or oxygen-breathing apparatus (SCBA) — the wearer carries an independent breathing supply, completely independent of the surrounding atmosphere, in one of two forms: open-circuit compressed-air units (a cylinder on the back, usually pressure-demand/positive-pressure, exhaled air vented to atmosphere), or closed-circuit oxygen rebreathers (compressed or chemically generated oxygen; exhaled CO₂ is scrubbed by an absorbent and the gas recirculated, giving longer duration of up to about 4 hours for a lighter unit, but with heat build-up and the fire risk of pure oxygen). Used for IDLH (immediately dangerous to life or health) atmospheres, oxygen-deficient spaces, unknown contaminants, and emergency/firefighting response. Its principal limitation is duration (typically 30–60 minutes for open-circuit air) and the added weight/bulk carried by the wearer.
• (b) Air-line breathing equipment — breathing air is supplied through a hose from a remote, fixed source (a compressor or cylinder bank) located outside the hazardous area, rather than carried by the wearer. This is lighter and permits much longer-duration work than SCBA, but tethers the worker to the supply hose (limiting mobility and creating a risk of hose damage, kinking, or snagging), and normally requires an auxiliary escape cylinder as backup in an IDLH atmosphere in case the air-line supply is interrupted.