Low Smoke vs Low Smoke Zero Halogen: What Engineers Need to Know

The terms "low smoke" and "zero halogen" describe two distinct properties of a material - and understanding this distinction is essential for proper specification.

A material can be low smoke without being zero halogen, or zero halogen without being low smoke. However, both properties are typically required together for safety-critical applications - which is why the term "low smoke zero halogen" (LSZH) has become the industry standard for environments where fire safety is paramount.

Why halogens matter

Traditional cables, particularly those insulated with PVC, contain chlorine - a halogen element. When PVC burns, it releases chlorine gas which combines with moisture to form hydrochloric acid. This creates two serious problems: the gas is poisonous and dangerous to inhale, and the resulting acid can cause devastating damage to nearby electronic equipment. According to the IET, LSF and LSZH cables are often confused to be the same - but they are fundamentally different in their halogen content and the standards to which they are tested (IET, 2024).

The confusion between LSF (Low Smoke and Fume) and LSZH (Low Smoke Zero Halogen) is one of the most common specification errors in cable installation. While both terms suggest improved fire safety, they represent significantly different levels of protection.

This lack of standardisation for LSF cables is a critical point. While LSZH cables must demonstrate compliance with specific test requirements, LSF cables have no defined performance criteria. An LSF cable from one manufacturer may perform very differently from another, making consistent specification impossible.

You will encounter various acronyms when specifying cables and labels for fire-safe installations. Understanding which terms are equivalent helps avoid specification errors.

Equivalent terms (interchangeable)

• LSZH: Low Smoke Zero Halogen - the most common UK/European term
• LSOH: Low Smoke Zero Halogen - alternative spelling, identical meaning
• LS0H: Low Smoke Zero Halogen - using zero rather than the letter O
• OHLS: Zero Halogen Low Smoke - reversed order, same specification
• LSHF: Low Smoke Halogen Free - common in some markets

Non-equivalent terms (different specification)

• LSF: Low Smoke and Fume - not guaranteed halogen-free, no defined standards
• FR: Flame Retardant - addresses flame spread only, not smoke or toxicity
• Plenum: US rating for air-handling spaces - may contain halogens

When reviewing specifications or placing orders, always verify the exact standard being referenced. A requirement for "low smoke cable" is ambiguous; a requirement for "cable compliant with BS EN 61034 and BS EN 60754" is precise and verifiable.

LSZH cables and labels are increasingly specified in environments where fire safety is critical. The common factor across all these applications is the need to protect people from toxic smoke and protect equipment from corrosive gases.

Public buildings and enclosed spaces

Hospitals, airports, supermarkets, shopping centres, schools, and museums all require careful consideration of cable fire performance. In these environments, large numbers of people may need to evacuate, making smoke density and toxicity critical factors. The Construction Products Regulation (CPR) requires cables fixed within building structures to demonstrate appropriate fire performance.

Transport infrastructure

Rail and underground systems have some of the most stringent requirements. Following the King's Cross fire in 1987, there was a significant move towards using LSZH materials across the London Underground network. The European standard EN 45545 now governs fire protection on railway vehicles across the EU and UK, specifying requirements for smoke, toxicity, and flame spread.

Data centres and control rooms

Beyond human safety, LSZH materials protect sensitive electronic equipment. The corrosive gases released by burning PVC can damage circuit boards, connectors, and other components throughout a facility - often extending damage far beyond the immediate fire location. Data centres increasingly specify LSZH for both cables and labels to minimise potential equipment losses.

Marine and offshore

Confined spaces on ships and offshore platforms make escape particularly difficult during fire events. LSZH cables and labels help ensure evacuation routes remain navigable by maintaining visibility and breathable air for longer periods.

Specifying LSZH cables while using standard labels creates a gap in your fire safety strategy. If the labels attached to compliant cables emit toxic smoke or corrosive gases, the overall installation no longer meets its intended specification.

This is not merely a theoretical concern. In environments like the London Underground, labels must meet the same fire performance requirements as the cables they identify. The LUL 1-085 standard covers all materials in the rail environment, including labels and markers.

Silver Fox® LSZH labelling solutions

For installations requiring low smoke zero halogen compliance, Fox-Flo® tie-on cable labels are manufactured from a premium LSZH material developed specifically for demanding environments. Fox-Flo® labels have been tested to meet London Underground fire safety requirements (LUL 1-085) and carry EN 45545-2 R22 certification for rail applications.

For heatshrink applications, Legend™ LSZH Heatshrink cable markers provide the same low smoke zero halogen properties in a shrinkable format. These markers are ideal for wire identification in public buildings, rail environments, and anywhere that requires both durable marking and fire safety compliance.

Both product ranges can be printed on-demand using the Fox-in-a-Box® thermal printer, allowing engineers to produce compliant labels at the point of installation rather than relying on pre-printed stock.

1. ?

   What is the low smoke zero halogen cable specification?

   LSZH cables must comply with BS EN 61034 for smoke emission and BS EN 60754 for halogen content. These standards define specific test methods and pass/fail criteria, unlike LSF cables which have no governing standards.

2. ?

   Is LSZH the same as LSOH?

   Yes. LSZH, LSOH, LS0H, OHLS, and LSHF all refer to the same specification: materials that produce low smoke and contain no halogens. The terms are used interchangeably across different markets and manufacturers.

3. ?

   Can I use standard labels on LSZH cables?

   Using standard labels on LSZH cables compromises the fire safety specification of the installation. In regulated environments like rail or underground systems, labels must meet the same fire performance requirements as the cables.

4. ?

   Is LSZH more expensive than standard materials?

   LSZH cables and labels typically cost more than PVC alternatives due to the specialised materials and testing required. However, in safety-critical environments the cost is justified by the protection provided to people and equipment.

5. ?

   What environments require LSZH?

   LSZH is typically required or strongly recommended in: public buildings (airports, schools, hospitals), transport systems (rail, underground, marine), data centres and control rooms, and any enclosed space where evacuation may be required during a fire event.

Selecting the right labelling materials for fire-safe installations requires understanding both the regulatory requirements and the practical options available. Silver Fox® has been manufacturing LSZH labels in the UK since developing the Fox-Flo® material specifically for demanding environments.

Our LSZH product range includes tie-on labels, heatshrink markers, and wrap-around labels - all tested to the standards that matter for rail, underground, marine, and public building applications. For more on selecting the right label type for your application, see our guide to choosing cable and wire labels for UK engineering projects.