• Fire and explosion: the vapours are highly flammable; in the flammable range with air they can be ignited by any ignition source (static, sparks, hot surfaces, smoking, electrical), causing fire or explosion.
• Toxicity and health hazard: crude oil and its vapours are toxic; inhalation causes dizziness, nausea, asphyxiation and long-term health damage; skin contact causes irritation and dermatitis.
• Asphyxiation: vapours displace oxygen in enclosed spaces (tanks, pump rooms), causing oxygen deficiency.
• Environmental pollution: spillage causes marine pollution, requiring containment and clean-up.
• Corrosion and material damage: crude oil and its sulphur content cause corrosion of tanks and piping.
• Static electricity and sloshing: during loading/cleaning, static charge can build up and ignite vapour.

• The International Safety Guide for Oil Tankers and Terminals (ISGOTT).
• The International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk (IBC Code) and the MARPOL Convention.
• The International Code of Safety for Ships Carrying Oil in Bulk (MARPOL Annex I).
• The ship's own Safety Management System (SMS) and the Cargo Manual, plus the International Maritime Dangerous Goods (IMDG) Code for packaged products.
• Company procedures and the tanker safety checklists (e.g. OCIMF Ship/Shore Safety Checklist).

An explosimeter measures the concentration of flammable vapour in air as a percentage of the lower flammable limit (LFL). Sketch: a hand-held instrument with a sampling tube, a pump (aspirator bulb or motor), a meter calibrated 0-100% LFL, and a sensing element (a heated platinum filament or catalytic bead) in a chamber.

Operation: the sample is drawn through the sampling tube into the instrument. The gas passes over a heated catalytic filament; if flammable vapour is present it burns on the filament, raising its temperature and changing its electrical resistance. This unbalances a Wheatstone bridge, producing a meter reading proportional to the vapour concentration, displayed as a percentage of the LFL. The instrument is calibrated against a known gas (e.g. pentane) and gives a reading; a reading above about 20-25% LFL indicates a dangerous condition and entry is prohibited. The instrument must be intrinsically safe (approved for use in flammable atmospheres) and used with a suitable sampling line lowered into the tank/pump room.

The lower flammable limit (LFL) is the minimum concentration of vapour in air below which the mixture is too lean to ignite; the upper flammable limit (UFL) is the maximum concentration above which the mixture is too rich to ignite. Between the LFL and UFL the mixture is flammable and will burn/explode if ignited. Sketch: a graph with the vertical axis as percentage of oxygen and the horizontal axis as percentage of hydrocarbon vapour (or a triangular diagram). The flammable range is the region bounded by the LFL and UFL lines and the oxygen line; the "too rich" region is above the UFL, the "too lean" region below the LFL, and the "oxygen deficient" region near the top where there is insufficient oxygen to support combustion. The graph shows that a mixture is only flammable within the shaded band between the LFL and UFL, and that inerting (reducing oxygen below about 8-11%) removes the flammable region entirely.