The paper establishes a quantitative link between the bulk hydrogen content and the local concentration in the core of the Σ9{2 2 1}[1 1 0] symmetric tilt grain boundary in Al. A detailed map of approximate segregation energies is obtained by combined semi-empirical and ab initio calculations. Even if the density of trap sites and the binding to the core are large, it is shown that segregation alone is not expected to lead to a significant loss of cohesion below 1000 ppm bulk concentration. Other mechanisms should be involved, like H-induced structural changes, to explain the experimentally observed failure of the interfaces at low H concentration. An example of such mechanism is reported.