The utilization stagewas chronologically the longest but had the
lowest impact in terms of emissions. This is convenient because it is
also the stage that is the most difficult to control given that the
product is scattered across dozens of locations. It is also easier for
the manufacturer to effect changes to the life cycle on site than for it
to expect all its customers to do so. Methane emissions were higher
in the disposal stage than in the manufacturing stage. This was to
be expected considering the asphalt roofing product was disposed
of in landfills. Another break in the trend was the emission of nitrogen
oxides. The reason the utilization stage had higher emissions
than the disposal stage was due to the transportation of the
product over vast distances to customers located across the United
States. While the product has to be transported to landfills at the
end of its life as well, the distance between a landfill and a customer
was, on average, a lot lower than the distance between a customer
and the manufacturing facility.