Carbon and low alloy steel notionally sweet pipelines could be in danger of sudden and/or severe cracking and, consequently, loss of containment if H2S is introduced into the media, even for a short time and/or at very low partial pressures.For mildly sour conditions i.e.Region 0 as defined in NACE MR0175/ISO 15156, the main threat is hydrogen induced cracking (HIC) and this needs to be considered even for trace amounts of H2S.The effect of long-term exposure at such low levels of H2S is not well defined and the window of safe operation for HIC resistance is yet to be established.Recent work carried out by the European Pipeline Research Group suggested that there is a threshold below which HIC may not be a credible threat.This work however used modern seam-welded linepipe, which is inherently cleaner and thus potentially less susceptible to HIC than heritage materials manufactured decades ago.The present paper is aimed at defining a threshold and assessing the applicability of the standard test methodology for the HIC resistance of heritage pipelines (i.e.manufactured decades ago).