I can start. Then I think my colleague David will want to jump in.
When we do a risk assessment, we assess whether the substance has negative impacts on human health and the environment. They are from a multitude of sources. They can be end-of-pipe industrial releases or diffused from products in landfills into the air, water, groundwater or surface water. They can come directly from the processes that produce products, from the products themselves, from the mining sector or pulp and paper. Anywhere there is the potential for pollution is a potential source of toxic substances.
I don't think there is one that is greater than another. When we look at each toxic substance, we do assess how the exposure is happening, and how humans or the environment are exposed to that particular substance. That allows us to target our risk management efforts, because we need to know the source of the substance in order to be able to address the risks and reduce them.