Gases and Vapours Flashcards
Odour
Odour threshold: established for many gases and vapours in occupational/environmental settings
Odour Concentration
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
Asphyxiants
Cause reduction in ambient O2 concentration by displacement or dilution
Most are odourless and cause accidental death - especially in confined spaces
Methane (CH4)
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
Carbon Dioxide (CO2)
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
Chemical Asphysiants
Affects the body’s breathing mechanism or the uptake distribution and use of O2 by the blood and/or tissue cells
Carbon Monoxide (CO)
Product of incomplete combustion of carbonaceous materials
Colourless and odourless gas
Leads to elevated carboxyhaemoglobin levels and reduced blood-oxygen carrying capacity
Hydrogen Sulphide (H2S)
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
Isocyanats
Can be liquids or solids at room temperature and are mainly used in the production of polyurethanes, foams, adhesives, varnishes and paints
Petroleum Products
Lubricating oils
Petrol
Gas, oils, fuel oils
Benzene
Specific Gravity
Ratio of density of material to density of water
Gas or Vapour Density Ratio
Ratio of density of a vapour/gas to density of air
Vapour Pressure
Vapours evaporate from liquids have a vapour pressure
Flammability
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
Explosive Limits
100% Lower Explosive Limit (100% LEL): the concentration of flammable gas in air below which an explosive gas atmosphere won’t be formed
Active Sampling
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
Activated Charcoal
Non-polar and preferentially absorbs organics (e.g. VOC)
Typically CS2 for desorption
Silica Gel
Used for polar substances (e.g. gluteraldehyde, amines, inorganics)
Disadvantages: affinity for water
Desorption: polar solvent such as water and methanol
Colorimetric Tubes
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
Passive Sampling
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
Gas chromatograph (GC)
Separates sampled compounds and quantifies against known standard samples
Mass Spectrometer (MS)
Usually combined with a GC as GC/MS, the GC separates and the MS identifies and quantifies unknown compounds
