MetFab - Welding Fume Handbook - Flipbook - Page 9
3 Gases
3.1 Formation/Composition
Gases can be present as by-products
of the welding or cutting process, as
shielding gases or as fuel.
The by-product pollutant gases
generated by welding are usually ozone,
oxides of nitrogen, carbon monoxide
and carbon dioxide. Welding in the
presence of solvent vapours or surface
coatings may also form organic vapours.
Shielding gases can be single chemical
species, typically carbon dioxide, argon
or helium, or mixtures containing
varying proportions of carbon dioxide,
argon and helium with minor additions
of hydrogen, oxygen, nitrogen or nitric
oxide.
Shielding gases may be hazardous to
health in their own right, generally
as asphyxiants. Fuel gases such as
acetylene and propane are flammable
and can form explosive mixtures with
air. Take care when storing, handling
and using shielding gases and fuel
gases, and refer to the appropriate
material safety datasheets (MSDSs) and
other safety literature available from
Air Products.
Ozone
Ozone is formed when ultraviolet (UV)
radiation from the arc splits oxygen
molecules in the surrounding air:
is readily absorbed in the air, so large
quantities of ozone are formed close
to the arc. This ozone enters the
rising plume of fume and gases and
is dissipated in the area around the
welding torch.
Radiation in the range 175-242 nm is
more penetrating, though less efficient
in producing ozone, so it generates
relatively small concentrations of
ozone in areas some distance from
the arc. Ozone is found in highest
concentrations in the immediate
area of the arc, and at much lower
concentrations some distance from
the arc. Ozone is only produced during
arcing, and decomposes rapidly when
the arc is extinguished.
Almost all welding processes generate
small amounts of ozone. Higher
concentrations occur during MIG/
MAG and TIG welding, especially of
aluminium and stainless steel.
Nitrogen oxides
Oxides of nitrogen are formed when
heat from the arc causes nitrogen
and oxygen in the air to react.
Different oxides are formed in varying
proportions, depending on the reaction
temperature.
Many welding and cutting processes
generate oxides of nitrogen, but not
usually in amounts that could pose
O + light (wavelength < 242 nm) > 20
a health hazard. Exceptions include
O + O > O
cutting and gouging, especially
The wavelength range 130-242 nm is the when using compressed air/nitrogen
most important for ozone formation,
mixtures, and other processes based on
and this range can be split into two
naked flames, such as preheating, flame
parts with different characteristics.
spraying and flame straightening. The
health risks are greatest during manual
In the range 130-175 nm, UV radiation
9
