Safe listening means enjoying music at levels and for durations that do not harm hearing. Hearing loss caused by loud sound is permanent and builds up gradually, and the World Health Organization estimates that more than a billion young adults are at risk of avoidable hearing loss from unsafe listening practices.
Risk depends on both loudness and time. The WHO-ITU standard for personal audio devices, published in 2019, uses a reference sound allowance of 80 dB for 40 hours a week for adults and 75 dB for 40 hours for children, and asks phones and players to track listening against it, warn users and offer volume limits. Louder listening uses up that allowance much faster.
Lo-fi is often played for hours while studying or falling asleep, so the practical rules are to keep the volume low, prefer well-fitting headphones that block outside noise rather than turning the level up, take breaks and use the listening-health features many phones already include. Ringing in the ears or muffled hearing after listening is a reason to see a doctor or audiologist.