Arising from: C. O'Dowd et al. Nature417, 632–636 (2002); see O'Dowd et al. reply.  O'Dowd et al. describe the formation of marine aerosols from biogenic iodine and the growth of these aerosols into cloud-condensation nuclei (CCN). Based on chamber and modelling results, the authors suggest that biogenic organic iodine compounds emitted from macroalgae may be responsible for coastal particle bursts and that production of these compounds in the open ocean could increase CCN there too. It has since been shown that coastal particles are more likely to be produced from the photooxidation of molecular iodine. Moreover, I contend that open-ocean particle production and cloud enhancement do not result from emissions of organic iodine at atmospheric levels. For iodine particles to affect cloud properties over the remote ocean, an additional source of iodine is necessary as organic precursors cannot be responsible.