Lead was historically the choice for fishing weights for a few reasons: it is cheap, it is dense, and it is malleable enough to pinch split shot securely to a line. The downside of lead is that it is toxic if ingested by wildlife. Whether directly or indirectly, lead has been shown to be the culprit with killing fish, mammals and birds (most famously, the common loon). Many states have outlawed the use of lead in fishing tackle.
The bottom line: there are many practical alternatives to lead including tin weights and tungsten putty. These are inert if left behind, and don’t have the toxic effects of lead.
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