What is a potential hazard of not using personal protective equipment when handling LPG or CNG?

The potential hazard of not using personal protective equipment when handling LPG (liquefied petroleum gas) or CNG (compressed natural gas) is indeed an increased risk of burns or inhalation of toxic gases.

When working with LPG or CNG, these fuels can be hazardous if there is a leak or if the equipment malfunctions. LPG is heavier than air and can accumulate in low areas, creating a risk of fire or explosion. Inhaling LPG can result in respiratory issues, while exposure to CNG in a confined area can lead to suffocation due to displacement of oxygen. Personal protective equipment (PPE) such as gloves, goggles, and face shields are critical in protecting individuals from these hazards.

The other options focus on mechanical or economic aspects rather than safety. Decreased fuel efficiency, inability to start the engine, and higher fuel costs do not directly relate to personal safety while handling these gases, thus making them less relevant in the context of this question.