Most lighting in a hazardous area is not chosen badly — it is marked correctly and read badly. An explosion proof luminaire carries a string of codes that tells you exactly where it may be installed, and nearly every specification error comes from reading one element of that string in isolation.
Start with the zone, because the zone decides everything else. Under the ATEX and IECEx systems, gas atmospheres are Zone 0, 1 or 2 and dust atmospheres are Zone 20, 21 or 22, depending on how often an explosive mixture is present. A fixture certified for Zone 1 may also serve in Zone 2, but never the other way round. North American practice divides the same reality into Class I/II and Division 1 or 2 — the systems are not interchangeable on paper even when the hardware looks identical, so the first check is always which system your site's area classification drawing uses.
Next, the gas group and the temperature class, which are a pair. Gas groups run from IIA (least easily ignited, such as propane) through IIB to IIC (hydrogen and acetylene), and a fixture approved for IIC covers the lower groups, while the reverse is not true. Temperature class is the other half: T6 guarantees the fixture's surface stays below 85 °C, T1 allows up to 450 °C, and the class must sit safely below the ignition temperature of the gases actually present. This is the context in which ranges of explosion proof lighting are normally organised — by zone, gas group and temperature class together, because a fixture is only ever suitable as a complete marking, not as a collection of separate best figures.
The protection concept explains how the fixture achieves its rating. Flameproof enclosures (Ex d) let an internal ignition happen and cool the escaping gases through engineered flame paths; increased safety (Ex e) prevents sparks and hot surfaces from arising in the first place; restricted breathing and pressurisation take other routes again. The concept matters in maintenance, not just in selection: an Ex d enclosure whose cover bolts are replaced with ordinary hardware, or whose flame path is scratched during relamping, is no longer the fixture its certificate describes.
Finally, the unglamorous figures that decide service life: ingress protection (IP65/IP66 is the working standard outdoors and in washdown areas), ambient temperature range — a fixture certified for −20 °C to +40 °C is not automatically valid on a −40 °C site or above a hot process line — and the quality of the cable gland interface, since the entry is part of the certified assembly. Read as a whole, the marking is a complete sentence: zone, group, temperature class, concept and ambient range. Specify against all five, and the fixture will match the area it hangs in.