Q: I’m a chef and often serve food to allergic customers. Is it safe for those with shellfish allergies to consume sea salt? I’ve read that it can contain a microscopic type of shellfish.
Dr. Sicherer: Sea salt differs from table salt primarily because sea salt is evaporated from seawater rather than refined from mined salt. Sea salt may come from different sources and these have different color, mouth feel and texture based on granule size and mineral content.
I believe what you’ve read about are the microscopic animals called marine copepods, which are related to shrimp. These minuscule animals are permitted in unfiltered public drinking water in areas that have excellent natural sources that require no filtrations.
No harmful effects have been related to these creatures, which would be consumed by large numbers of individuals drinking public water (e.g. in New York City).
Copepods likely have shellfish-allergy relevant proteins, but the amount of exposure should be too low to trigger allergic reactions.
The dilution effect of water would presumably reduce exposure and while concentrating seawater to create sea salt may theoretically concentrate copepods if they are present, I could locate no studies or scientific reports to verify allergic reactions to sea salt or unfiltered water attributable to copepods. The risk appears to be extremely low.
Shellfish and Fish Allergies Explored 
Dr. Scott Sicherer is Chief of the Division of Allergy and Immunology of the Jaffe Food Allergy Institute at the Icahn School of Medicine at Mount Sinai in New York. Together with Dr. Hemant Sharma, Associate Chief of the Division of Allergy and Immunology at Children’s National Medical Center in Washington, he writes “The Food Allergy Experts” column in the American Edition of Allergic Living magazine. Questions submitted below will be considered for answer in the magazine.