The journal impact factor (JIF) for The Annals of Occupational Hygiene rose 68% between 2005 and 2006. JIFs are widely publicized and may influence subscriptions and where authors submit papers, so they are much discussed in the publishing world. But although they tell us something about a journal's citation performance, their shortcomings mean that they are poor general indicators of journal quality, and worse guides to the quality of authors and their institutions. The shortcomings include the following. (i) The two cited years may completely misrepresent the total current citation rate for the journal. (ii) The short citation period (1 year) results in many papers not contributing to the JIF, and usually two-thirds or more of the JIF depend on the most-cited 25% of papers. (iii) The JIF of the journal where a paper is published is therefore a very poor guide to the paper's citation performance or the success of the author. Citation counts more specific to the author are much better. (iv) The JIF depends strongly on the subject of the journal, even within the published categories. (v) Statistical analysis shows that the relative standard deviation of year-to-year variation of a JIF for a journal with a JIF approximately 1.5 is likely to be between 10 and 20%, on top of any longer trend. Quotation of JIFs to three decimal places is therefore meaningless, and, for a journal like Annals of Occupational Hygiene, a single annual change of 70% could easily be due to a chance shift from a negative to positive fluctuation. (vi) The citations counted are not only of individual papers, so it is difficult to reproduce the JIF calculation. (vii) The selection of journals has been criticized, for example, the alleged emphasis on American- or English-language publications. This journal's JIF does not noticeably influence the number of papers submitted to this journal, although it may influence some important authors. JIFs in our field seem to be increasing by approximately 5% a year, perhaps partly because of the various measures which editors can take to improve them, some of which represent genuine improvements to the journal, but some of which are unethical.