Public health is the application of health science in a community context. It's science, but it's also politics. It follows that the information needs are very broad and overlap with subject areas that we don't usually think of as being health-related. Like their clinical counterparts, though, most public health workers will find PubMed to be an enormously useful tool and a great boon to their practice. But not all will.
Environmental health is a good example of an information challenge encountered in public health.
The point is not to be discouraging. A librarian trained in working with health professional information needs has a tremendous amount to offer public health workers. Just be aware that many needs, and the resources to meet them, go beyond those that most of us are familiar with. No need to be daunted by that. As a librarian, you have information retrieval and evaluation skills that are of great value to this audience.