Flammable and combustible liquids

Flammable and combustible liquids are the most common hazardous chemicals in workplaces. Gasoline, diesel, solvents, thinners, paints, adhesives, alcohols, degreasers and many cleaning products fall into this group. Because they are so familiar, people forget how much energy they hold: a single litre of gasoline vapour mixed with air can level a room. This lesson explains how these liquids are classified, why the classification systems differ between hazard communication and fire codes, how quantity limits work, and how to keep ignition sources and vapours apart. Basic fire chemistry, such as the fire triangle and flash point, is covered in HSE-250 and is only reviewed briefly here.

Flash point, vapour and the flammable range

A liquid does not burn; its vapour does. The flash point is the lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture with air near its surface. A liquid stored above its flash point is always producing a flammable vapour layer. Gasoline has a flash point of about −43 °C (−45 °F), so it is above its flash point in any climate where people work. Diesel fuel has a flash point of about 52 °C (126 °F) or higher, so at normal room temperature it gives off little vapour, but it will burn readily once heated or sprayed as a mist.

Vapour will only ignite when its concentration in air is between the lower flammable limit (LFL, also called the lower explosive limit or LEL) and the upper flammable limit (UFL). For many common solvents the LFL is around 1 to 2 per cent by volume, which is a very small amount: a 1 L spill of acetone that evaporates completely in a small closet can easily produce a flammable atmosphere. Most solvent vapours are heavier than air and spread along the floor, collecting in pits, drains and low areas, and they can travel a long way to an ignition source and flash back to the container.

Two classification systems

Flammable liquids are classified in two different ways, and storage work needs both. Hazard communication law (WHMIS in Canada and the OSHA Hazard Communication Standard in the United States) uses the GHS categories, which appear on labels and in SDS section 2. Fire codes use an older system of classes based on flash point and boiling point. The two systems use different cut-off temperatures, so a product's GHS category does not always map neatly onto a fire-code class.

In practice, find the flash point and boiling point in SDS section 9, then classify the product under both systems. GHS Category 3 is the tricky one: it covers liquids with flash points from 23 °C to 60 °C (73 °F to 140 °F), which straddles the fire-code line between flammable Class IC and combustible Class II. A product with a flash point of 30 °C (86 °F) is a Class IC flammable liquid under the fire code; one with a flash point of 45 °C (113 °F) is a Class II combustible liquid. Both are GHS Category 3.

In Canada, the classes are defined in the National Fire Code of Canada and the provincial codes based on it, mainly in Part 4, Flammable and Combustible Liquids. The National Fire Code 2020 was the current national edition when this course was checked in October 2026, and provinces adopt editions on their own schedules, so check which edition applies to your site.

How quantity limits work

Fire codes limit how much flammable and combustible liquid can be stored in a given area, because the more fuel there is, the faster a fire grows and the harder it is to control. The limits depend on the class of liquid, the type of occupancy (an office, a school, a factory, a warehouse, a laboratory), whether the area is sprinklered, and whether the liquid is in cabinets, safety cans or a storage room. Limits are often expressed as a maximum allowable quantity per control area or fire compartment, with increases allowed for sprinklers and for storage in approved cabinets.

Container type also matters. Codes set maximum container sizes for each class, with smaller limits for glass than for metal or approved safety cans. A safety can is a metal or approved plastic container with a spring-closing lid, a spout cover and a flame arrester screen that stops a flame from travelling into the can. For dispensing small amounts of Class I liquids, safety cans are much safer than glass bottles or open containers.

The code tables are long and differ between jurisdictions and editions, so this course does not reproduce them. Instead, it gives a method: list what you store by class, total it by area, compare it with the limits in the code that applies, and reduce or relocate stock until you are within the limits with a margin.

# Tally flammable and combustible liquid stock by fire-code class for one area.
# Invented inventory for a small college shop and lab. Volumes in L.
L_PER_GAL = 3.785
stock = [
    ("acetone",          "IB",   4 * 4.0),
    ("gasoline",         "IB",   2 * 20.0),
    ("isopropyl alcohol", "IB",  6 * 4.0),
    ("toluene",          "IB",   2 * 4.0),
    ("ethanol 95%",      "IB",   4 * 4.0),
    ("mineral spirits",  "II",   4 * 20.0),
    ("diesel",           "II",   1 * 20.0),
    ("hydraulic oil",    "IIIB", 4 * 20.0),
]
totals = {}
for name, cls, vol in stock:
    group = "Class I (flammable)" if cls in ("IA", "IB", "IC") else ("Class II" if cls == "II" else "Class III")
    totals[group] = totals.get(group, 0) + vol
for g, v in totals.items():
    print(f"{g:22s} {v:6.0f} L  ({v / L_PER_GAL:5.1f} gal)")
flam = totals["Class I (flammable)"]
print(f"Class I in safety cans or cabinet? {flam:.0f} L must be checked against the code limit")

Class I (flammable)       104 L  ( 27.5 gal)
Class II                  100 L  ( 26.4 gal)
Class III                  80 L  ( 21.1 gal)
Class I in safety cans or cabinet? 104 L must be checked against the code limit

The tally shows that this invented area holds about 104 L (about 27 gal) of Class I flammable liquids, 100 L (26 gal) of Class II combustibles, and 80 L (21 gal) of Class IIIB oils. The next step would be to compare each total with the limit for the occupancy and sprinkler status in the code that applies, and to move the gasoline to an outdoor storage location or a cabinet, since it is the most hazardous product and the one least needed indoors.

Controlling ignition sources

The second half of flammable liquid safety is keeping vapours away from ignition. Common ignition sources in workplaces include open flames (torches, pilot lights, space heaters), hot surfaces (engines, ovens, steam pipes, light bulbs), electrical sparks from switches, motors and extension cords, static electricity discharges, friction and grinding sparks, and smoking. Inside storage rooms and around dispensing, electrical equipment must be suitable for the hazardous location, which is classified under the Canadian Electrical Code or the US National Electrical Code. Static electricity and bonding are covered in lesson 9.

Keep containers closed when not in use. An open container of a Class I liquid is a continuous vapour source. Store rags soaked with solvents or oils in an approved self-closing oily waste can, emptied daily, because oily rags, especially those with linseed oil or oil-based stains, can heat up and ignite by themselves.

Fires involving flammable liquids

Flammable liquid fires spread fast and can flow across floors and down drains. Water from a hose can spread a liquid fire. Portable extinguishers rated for Class B fires (dry chemical, carbon dioxide or foam) are the right tools for small fires, and only for people trained to use them. For larger storage, foam suppression or specially designed sprinklers may be required by the fire code. The best fire protection is prevention: small quantities, closed containers, good ventilation and strict control of ignition sources.

Aerosols

Aerosol cans of paints, lubricants, brake cleaners and adhesives are easy to overlook in a flammable liquid inventory, but most contain flammable propellants such as propane and butane as well as flammable solvents. In a fire, aerosol cans can rupture and rocket across a room, spreading fire well beyond the original area. Fire codes classify aerosol products by their heat of combustion into levels and set separate quantity limits and storage rules for them. Store aerosols in a flammable cabinet or in a wire-mesh aerosol cage, away from heat, and count them in your inventory. Punctured or empty cans may still contain propellant and solvent, so dispose of them through the waste program rather than in general trash, unless your local rules say otherwise.