What Does cal/cm² Mean?

The rating cal/cm² represents how much heat energy is delivered to a specific area of the body. It accounts for:

• The intensity of the heat source
• The duration of exposure
• The surface area impacted

Because burn injuries depend on all three factors, cal/cm² is the standard unit used to evaluate FR clothing performance.

Why 1.2 cal/cm² Matters for Worker Safety

A key benchmark in thermal protection is 1.2 cal/cm², which is widely recognized as the threshold for a second-degree (blister) burn on unprotected skin.

To put that into perspective:

• Holding the hottest part of a lighter flame under your palm for about one second can result in roughly 1.2 cal/cm² of exposure
• This is the point where a burn becomes more serious and can lead to blistering and longer recovery

Importantly:

• 1.2 cal/cm² does not mean “no injury”
• It means the injury is more likely to be recoverable without permanent damage or loss of mobility

That’s why FR and arc-rated PPE are designed to reduce heat transfer to below this level, helping prevent life-altering injuries.

Understanding Arc Ratings in FR Clothing

An arc rating indicates how much thermal energy a garment can protect against, measured in cal/cm².

For example:

• If a task exposes a worker to 6 cal/cm², the worker should wear clothing rated 6 cal/cm² or higher

The goal is not to eliminate all heat exposure—but to limit the energy reaching the skin to below the 1.2 cal/cm² injury threshold.

Arc Flash vs. Flash Fire: Key Differences

Not all thermal hazards behave the same way:

Arc Flash

• Extremely high heat
• Very short duration (fractions of a second)

Flash Fire

• Lower heat intensity
• Longer duration (up to ~3 seconds)

Because of these differences:

• An arc-rated garment does not automatically provide full flash fire protection
• PPE must be selected based on the specific hazard type, not just the calorie rating

How Heat Exposure Is Calculated

Thermal hazard exposure is based on:

• Energy level (cal/cm²)
• Duration of exposure
• Distance from the heat source

For example:

• Arc flash energy is calculated at a specific working distance (e.g., 15 inches)
• Flash fire exposure considers energy over time

These factors determine the minimum level of protection required.

Choosing the Right FR Clothing

To properly protect against burn injury:

1. Identify the hazard (arc flash or flash fire)
2. Determine the expected exposure level in cal/cm²
3. Select garments with an arc rating that meets or exceeds that level

Never assume one rating covers all hazards—protection must match the risk.

Final Takeaway

A calorie in FR clothing is a measure of heat energy exposure, and it plays a direct role in worker safety.

Understanding cal/cm² and the 1.2 cal/cm² burn threshold helps ensure PPE is selected to minimize injury severity. While FR clothing may not prevent all burns, it is designed to reduce injuries to a survivable and recoverable level, making it a critical last line of defense in high-risk environments.