Abstract

The purpose of this study was to assess the effect of humidity and packaging materials on the stability of iodine in typical salt samples from countries with tropical and subtropical climates, under controlled climatic conditions. Initially we examined eight samples. In the second phase we expanded the study to salts from 18 sources and attempted to correlate the observed stability with salt impurities naturally present in these samples. High humidity resulted in rapid loss of iodine from salt iodized with potassium iodate, ranging from 30% to 98% of the original iodine content. Solid low-density polyethylene packaging protected the iodine to a great extent. High losses were observed from woven high-density polyethylene bags, which are often the packaging material of choice in tropical countries. Impurities that provided moisture at the salt surface had the most deleterious effect. Although clear correlations were not obtained, the presence of reducing agents, hygroscopic compounds of magnesium, and so forth seemed to have the most adverse effects on the stability of iodine. Surprisingly, carbonates had little effect on stability over the range present in the samples. Packaging salt in low-density polyethylene bags, which provided a good moisture barrier, significantly reduced iodine losses, and in most cases the iodine content remained relatively stable for six months to a year. The findings from this study indicate that iodine can be highly unstable, and in order to ensure the effectiveness of local salt-iodization programmes, countries should determine iodine losses from local iodized salt under local conditions of production, climate, packaging, and storage.