Why do spots appear on iceburg lettuce during cold storage？（0~4℃）
The brown spots you see are known as "russet spotting". This is a reaction which is believed to be caused by ethelyne gas emitted from citrus, such as apples, bananas, etc.

Bottom line, the lettuce is completely safe to eat. It's certainly less visually appealing, but it is indeed safe to eat. For ideas on preventing russet spotting, check out the source linked above.