Book Extract from Mycelium Running: How Mushrooms Can Help Save the World, 2005,
by Paul Stamets. Published by 10 Speed Press, Berkeley.

Page 110:

Most mushrooms do not actively concentrate lead above the level of concentration in the environment, with a few exceptions. One such exception, the morel, proliferates in burned habitats where fire has reduced the bulk of organic matter, an event that increases baseline metals in the “soil”. Lead is made less soluble, and hence less extractable, in soils where the pH is near neutral; adding lime (calcium carbonate) significantly reduces the solubility of lead, cadmium and other metals, thus locking them up and reducing infiltration into water and/or living organisms. In contrast, acidic soils allow lead to be easily absorbed by plants and mushrooms. According to Garcia and others (1998), saprophytic mushrooms absorb more lead than mycorrhizal ones, shaggy manes, especially those found in cities, could be bioindicators of lead contamination. Naturally, one wonders if the very presence of shaggy manes could indicate lead contamination? I don’t know, but I am suspicious when I find shaggy manes near industrial sites and avoid eating them.

…Mushrooms growing along roadsides, particularly where there is exposure to leaded gasoline [leaded petrol], should not be eaten.