Prompted by a question about the impact of dietary iron in elves (How would Fair Folk-type elves deal with dietary iron?), I wondered whether haemocyanin could be an alternative to haemoglobin in creatures, such as elves, who have a weakness to iron.

Although - as far as I know - only some arachnids, arthropods, and molluscs have haemocyanin, I'm making the initial assumption that it is in fact possible for human-sized beings to have copper-based blood. With that in mind, what sort of impacts (positive or negative) would this blood chemistry have on humanoids?

My thoughts so far:

1. Haemocyanin is second only to haemoglobin in frequency as an oxygen transport molecule. Is it any more/less effective? If it is less effective, would that result in these creatures living at lower altitudes with greater atmospheric oxygen?
2. The obvious colour difference: deoxygenated blood is clear, and oxygenated blood is blue.
3. Diseases like anaemia would probably have parallels - could this be treated easily with supplementary dietary copper (in mild cases)?
4. Cross-breeding with humans would become difficult, I expect - half-elves wouldn't exist, and heterozygosity could be compromised.

Prompted by a question about the impact of dietary iron in elves (How would Fair Folk-type elves deal with dietary iron?), I wondered whether haemocyanin could be an alternative to haemoglobin in creatures (for example elves, who have a weakness to iron - although probably not haemoglobin).

Although - as far as I know - only some arachnids, arthropods, and molluscs have haemocyanin, I'm making the initial assumption that it is in fact possible for human-sized beings to have copper-based blood. With that in mind, what sort of impacts (positive or negative) would this blood chemistry have on humanoids?

My thoughts so far:

1. Haemocyanin is second only to haemoglobin in frequency as an oxygen transport molecule. Is it any more/less effective? If it is less effective, would that result in these creatures living at lower altitudes with greater atmospheric oxygen?
2. The obvious colour difference: deoxygenated blood is clear, and oxygenated blood is blue.
3. Diseases like anaemia would probably have parallels - could this be treated easily with supplementary dietary copper (in mild cases)?
4. Cross-breeding with humans would become difficult, I expect - half-elves wouldn't exist, and heterozygosity could be compromised.

EDIT

I'm not too bothered about why they might have copper-based blood, but more about the implications if they do have it. Thank you for pointing out that haemoglobin wouldn't actually be toxic/harmful to an elf - it's not exactly iron filings floating around in the bloodstream. ;)