Properties of gasoline, diesel and other fuels

Every safety rule at a fuel station follows from a few physical facts about the fuels. Gasoline gives off flammable vapor even on the coldest day. The vapor is heavier than air, so it flows along the ground and collects in low spots. It only burns when it is mixed with air in a narrow range of concentrations, but that range is easy to reach near an open fill port, a spill or an overflowing tank. Diesel is much harder to ignite but still burns fiercely once it does. This lesson explains these properties in plain terms, so the rules in the rest of the course make sense rather than being a list to memorize.

HSE-330 covers flammable atmospheres in depth for oil and gas work, and HSE-105 covers how fires start and behave. This lesson focuses on the fuels a station handles: gasoline, diesel, ethanol blends, diesel exhaust fluid, and the propane and lubricants sold in the store.

The fire triangle and the fuels

A fire needs three things: fuel, oxygen and an ignition source. At a fuel station, oxygen is everywhere and fuel is the product, so most of the safety effort goes into two things: keeping fuel and vapor contained, and keeping ignition sources away from any vapor that escapes. Liquid fuel itself does not burn; it is the vapor above the liquid that ignites. That is why the properties that matter most are the ones that describe the vapor.

• Flash point: the lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air at its surface. Below the flash point, the vapor is too thin to ignite; above it, a spark can light it.
• Flammable range (explosive limits): the range of vapor concentrations in air that can burn. Below the lower flammable limit (LFL, also called the lower explosive limit, LEL), the mixture is too lean; above the upper limit (UFL or UEL), too rich. Between them, it burns.
• Vapor density: the weight of the vapor compared with air. A vapor density above 1 means the vapor is heavier than air and sinks.
• Autoignition temperature: the temperature at which the vapor ignites without a spark or flame, for example on a very hot exhaust part.

Gasoline

Gasoline is a blend of hundreds of hydrocarbons, most of them light enough to evaporate readily. Its flash point is −45 °F (about −43 °C), so it gives off ignitable vapor in any weather a station will see. Its flammable range is narrow, roughly 1.4% to 7.6% vapor in air, but the lower limit is low: only about one part in seventy of the air needs to be gasoline vapor to burn. Gasoline vapor is about three to four times heavier than air. It spills off the edge of a fill port, rolls across the forecourt, and collects in drains, pits, low ground and the inside of vehicles and containers. Its autoignition temperature is roughly 250 °C to 300 °C (480 °F to 570 °F), low enough that a hot exhaust or catalytic converter can light a pool of spilled gasoline.

Because of these properties, gasoline is a Class I flammable liquid in fire codes: the most closely controlled class. In WHMIS and in the Hazard Communication Standard it is classified as a flammable liquid category 1 and carries health hazard classes as well, because it contains benzene and can cause cancer, genetic damage and lung damage if swallowed and inhaled into the lungs (Lesson 4).

Diesel

Diesel fuel is heavier and less volatile than gasoline. Its flash point is above 126 °F (52 °C) for the common grades in Canada and the United States, so at normal temperatures it does not give off enough vapor to ignite with a spark. Fire codes therefore class it as a combustible liquid rather than a flammable one. That does not make it harmless. Diesel heated above its flash point, sprayed as a fine mist from a leaking high-pressure line, or soaked into a rag or absorbent can burn readily, and a diesel fire is hard to put out. Diesel on a forecourt is also very slippery and is a major cause of slips, and it irritates the skin with repeated contact.

Some stations sell a winter diesel blended with kerosene to keep it flowing in the cold. Kerosene's flash point is lower than diesel's, so winter blends may have a lower flash point than summer diesel, though still well above typical air temperatures.

Look closely at the autoignition temperatures. Diesel's is lower than gasoline's. Diesel is hard to light with a spark because it gives off little vapor, but once its vapor reaches a hot surface it can ignite without a spark at all. This is why fuel leaking onto a hot engine or exhaust is dangerous whatever the fuel.

Ethanol blends

Most gasoline sold in the United States is E10, which contains up to 10% ethanol. Many stations also sell E15 and some sell E85 for flex-fuel vehicles. Summer sales of E15 have depended on a series of emergency fuel waivers from the EPA; in 2026 the EPA again issued nationwide summer waivers, while bills to allow E15 year-round were still pending in Congress at the time of writing. Ethanol mixes with water, so ethanol blends are more sensitive to water in tanks, and ethanol fires burn with a pale, almost invisible flame in daylight, which makes them harder to see. Firefighters use alcohol-resistant foam on large ethanol fires, because ordinary foam breaks down.

For the cashier, ethanol blends change little in daily work. Spills are handled the same way, and the same ignition rules apply. The main points are to make sure customers use the right nozzle, since E85 and E15 must not go into vehicles that are not designed for them, and to remember that an ethanol fire may be hard to see.

Other products at the station

• Diesel exhaust fluid (DEF): a solution of urea in water used by modern diesel engines to reduce emissions. It is not flammable and has low toxicity, but it corrodes some metals, is slippery when spilled, and crystallizes into a white crust. It must never go into a fuel tank, and fuel must never go into a DEF tank; both mistakes cause expensive engine damage.
• Propane: sold in cylinders through an exchange cage. Propane is a gas stored as a liquid under pressure; its vapor is heavier than air and very flammable. HSE-353 covers propane in detail.
• Motor oil, antifreeze and windshield washer fluid: sold in the store. Windshield washer fluid is usually methanol and water and is both flammable and toxic if swallowed; antifreeze is toxic and tastes sweet, a danger to children and pets.
• Kerosene and heating oil: sold from a separate dispenser at some rural stations, into approved containers.

Where vapor goes

Because gasoline vapor is heavier than air, it behaves almost like a liquid you cannot see. It spills over the rim of a tank opening, flows downhill and pools in the lowest places nearby. Wind thins it out and carries it away, but on a still day it can travel several meters across the ground before it mixes enough to fall below the lower flammable limit. This is why the ignition rules apply to an area around the dispensers and fill ports, not just to the nozzle itself, and why drains, pits, sumps and basements near a spill are so dangerous.

The program below takes vapor readings from a hypothetical gas detector carried around a small spill and checks each against gasoline's flammable range. Detectors used for flammable gas read in percent of the lower explosive limit (%LEL), not percent vapor, so the program converts between the two. A reading of 100% LEL means the mixture has reached the lower flammable limit. Common alarm settings are 10% and 20% LEL; the readings are illustrative.

# Convert gas detector readings (%LEL) to % fuel in air and check against gasoline's flammable range.
LFL, UFL = 1.4, 7.6          # % gasoline in air (typical values)
readings = [("at the fill port opening", 160), ("on the ground beside the spill", 85),
            ("at the drain grate", 120), ("1 m above the spill", 12), ("at the store door", 0)]
for place, lel_pct in readings:
    fuel = lel_pct / 100 * LFL
    if fuel >= UFL:
        state = "too rich, but can become flammable as it mixes"
    elif fuel >= LFL:
        state = "FLAMMABLE"
    elif lel_pct >= 20:
        state = "high alarm: evacuate area, no ignition sources"
    elif lel_pct >= 10:
        state = "low alarm: caution"
    else:
        state = "below alarm"
    print(f"{place:34}{lel_pct:>5}% LEL = {fuel:4.2f}% in air  {state}")

at the fill port opening            160% LEL = 2.24% in air  FLAMMABLE
on the ground beside the spill       85% LEL = 1.19% in air  high alarm: evacuate area, no ignition sources
at the drain grate                  120% LEL = 1.68% in air  FLAMMABLE
1 m above the spill                  12% LEL = 0.17% in air  low alarm: caution
at the store door                     0% LEL = 0.00% in air  below alarm

The readings tell the story of heavy vapor. Right at the fill port and at the drain, the mixture is above the lower flammable limit and would ignite with a spark. On the ground beside the spill it is close to the limit and well past any alarm. A meter above the spill it is far lower, and at the store door the detector reads nothing. A person standing upright might smell fuel but feel safe, while the vapor at their feet and in the drain is ready to burn. Most stations do not carry gas detectors; the lesson here is about behavior: treat the ground around a spill and any drain nearby as the danger zone, and keep ignition sources well away.

For supervisors: read the SDS with your team

The safety data sheet for gasoline and diesel from your supplier lists the exact flash point, flammable limits, exposure limits and first aid for the products you sell. Sit down with new staff and go through sections 2 (hazards), 4 (first aid), 5 (firefighting), 6 (accidental release) and 9 (physical properties). Point out the numbers in this lesson on the actual sheet. It takes fifteen minutes and makes the SDS a working tool rather than a binder on a shelf.