For some fumes non metal duct of the correct specification may be acceptable.

The properties of the fume concerned needs to be checked for compatibility with the specific duct construction (i.e. temperature, corrosion, fire risk etc).

There are three main reasons why it is not suitable, these are: 

(1) Risk of Static generated sparks igniting an explosion.
The 2 types of “Plastic” ductwork, UPVC & PP, generate static which can spark and ignite fumes. You must check if the substance concerned is flammable and/or potentially explosive in either fume form or its liquid state in case it condenses to liquid in the duct. Explosive atmospheres legislation usually requires that ductwork for potentially explosive substances must be manufactured from a conductive material, i.e. Steel, so that any static electricity generated can be discharged to earth.

(2) Plastic Ductwork is usually designed for low velocities
Principles of system design specify a minimum air velocity necessary to keep fumes airborne UPVC and PP ducting is designed to be used with a system where the air velocity is less than 10 m/sec.  Some extraction systems will have been designed with higher velocities which could lead to the duct leaking.

(3) Chemical Damage
Chemical compatibility is very important - some chemicals will damage the plastic duct causing leaks. You must check with the duct manufacturer if the plastic ducting is guaranteed to be resistance to the specific chemicals in use at the process temperature, as a fume and also in liquid form as there may be potential for this to condense back to the liquid state inside the duct and pool. Also check the MSDS for the chemical to see how it should be stored - this may caution against storage in plastic.

UPVC and PP technical information:

Electrical Classification - an insulator. 

The surface resistivity of both UPVC and PP is greater than 10¹⁴ ohms and electrostatic charge can therefore accumulate easily.

Fire Characteristics

• UPVC:  Burns when in contact with a flame, giving off black smoke.  Stops burning on extinction of the flame, and evolves white fumes for a time.  The products of combustion are noxious.

• PP: Ignites readily and melts and drips as it burns. Gives off black smoke on burning, the quantity depending on the fire conditions.

YOU MUST CHECK THE MANUFACTURER’S SPECIFICATION FOR:

• RECOMMENDED MAXIMUM AIRFLOW VELOCITY

• CHEMICAL RESISTANCE

• STATIC DISCHARGE PROPERTIES

• COMBUSTIBILITY & BURNING CHARACTERISTICS