Gases and Vapours

Question 1

Odour

Answer 1

Odour threshold: established for many gases and vapours in occupational/environmental settings

Question 2

Odour Concentration

Answer 2

The dilution ratio at which the odour threshold level is reached during olfactory testing
- odour threshold: in olfactory analysis, 50% of panelist respond to odour and 50% don’t

3 different threshold levels used in environmental odour measurement

• guessing threshold
• detection threshold
• recognition threshold

Hedonic tone: degree to which odour is perceived as pleasant or unpleasant
Odour character: what odour smells like

Question 3

Asphyxiants

Answer 3

Cause reduction in ambient O2 concentration by displacement or dilution

Most are odourless and cause accidental death - especially in confined spaces

Question 4

Methane (CH4)

Answer 4

Found in fossil fuels and coal mines
Present in sewers and wherever biodegradable matter collects, is stored or dumped
Flammable, explosive and lighter than air in enclosed spaces where ventilation is poor, is less dense than air

Question 5

Carbon Dioxide (CO2)

Answer 5

OES: 5000 ppm; TWA: 30000 ppm
High CO2 affects blood pH and stimulates the respiratory centre causing rapid breathing at >10% loss of consciousness
Heavier than air when concentrated and ‘pools’ in low enclosed, unventilated places such as dumps and sewers

Question 6

Chemical Asphysiants

Answer 6

Affects the body’s breathing mechanism or the uptake distribution and use of O2 by the blood and/or tissue cells

Question 7

Carbon Monoxide (CO)

Answer 7

Product of incomplete combustion of carbonaceous materials
Colourless and odourless gas
Leads to elevated carboxyhaemoglobin levels and reduced blood-oxygen carrying capacity

Question 8

Hydrogen Sulphide (H2S)

Answer 8

A colourless, flammable, poisonous gas that smells like rotten eggs at lower concentrations - at higher concentrations sense of smell is eliminated
A neurotoxic chemical asphyxiant and affects the brain’s breathing control centre

Question 9

Isocyanats

Answer 9

Can be liquids or solids at room temperature and are mainly used in the production of polyurethanes, foams, adhesives, varnishes and paints

Question 10

Petroleum Products

Answer 10

Lubricating oils
Petrol
Gas, oils, fuel oils
Benzene

Question 11

Specific Gravity

Answer 11

Ratio of density of material to density of water

Question 12

Gas or Vapour Density Ratio

Answer 12

Ratio of density of a vapour/gas to density of air

Question 13

Vapour Pressure

Answer 13

Vapours evaporate from liquids have a vapour pressure

Question 14

Flammability

Answer 14

Properties to consider when hydrocarbon solvents are used:

• FLASH POINT: lowest temperature solvent will give off vapour in test vessel that will form an ignitable mixture with air
• AUTO IGNITION TEMPERATURE: temperature a flammable gas/air will ignite spontaneously

Question 15

Explosive Limits

Answer 15

100% Lower Explosive Limit (100% LEL): the concentration of flammable gas in air below which an explosive gas atmosphere won’t be formed

Question 16

Active Sampling

Answer 16

Collection of airborne hazards with an air sampling pump and appropriate sampling media
Media depends on the contaminant:
- solid sorbents
- liquid solutions
- bag or canisters
Used when:
- concentration constant
- measuring peaks or short periods

Question 17

Activated Charcoal

Answer 17

Non-polar and preferentially absorbs organics (e.g. VOC)

Typically CS2 for desorption

Question 18

Silica Gel

Answer 18

Used for polar substances (e.g. gluteraldehyde, amines, inorganics)
Disadvantages: affinity for water
Desorption: polar solvent such as water and methanol

Question 19

Colorimetric Tubes

Answer 19

Air drawn through glass tube with a hand pump
Contaminant reacts with chemicals to give a colour stain chance read off the scale as ppm
Advantages: immediate results, range of tubes, not expensive, can be used for walk throughs/spot checks
Disadvantages: limited shelf life, less accurate, tubes

Question 20

Passive Sampling

Answer 20

Collection of airborne gases and vapours at a rate controlled by diffusion through air or permeation through a membrane without an air sampling pump
Rely on molecular diffusion of gases and vapours
Used for: VOCs, ammonia, formaldehyde ethyl oxide

Question 21

Gas chromatograph (GC)

Answer 21

Separates sampled compounds and quantifies against known standard samples

Question 22

Mass Spectrometer (MS)

Answer 22

Usually combined with a GC as GC/MS, the GC separates and the MS identifies and quantifies unknown compounds