It's not good. But there are two scenarios:
- there may have been a suspect area of the media which couldn't be read, and the disk may have already reallocated those sectors somewhere else. In that case, the raw rate should be stable and not changing.
- there may be an issue with the heads which is causing ongoing read errors. In that case the raw rate will keep increasing.
The latter would be a lot more cause for concern. In either case I'd do as Wamphryi suggests and make sure you have another copy of the data just to be safe.