Why is the blood alcohol 08 percent?

There is no evidence to suggest that the legal blood alcohol limit is universally set at 0.8%. In most countries, it ranges from 0.02% to 0.08%. For instance, In Australia it's 0.05%, while it can be as low as 0.02% in Sweden and as high as 0.08% in the Czech Republic.

0.08% specifically might have caught on as a benchmark in some regions due to influential precedents or cultural factors rather than any universal scientific reasoning. To accurately answer this question, one would need specific context regarding which jurisdiction or country you're referring to.