Motion sickness experiences were obtained from participants in a 9 month, round the world yacht race. Race participants completed questionnaires on their motion sickness experience 1 week prior to the start of the race, during the race, and following the race. Yacht headings, sea states, and wind directions were recorded throughout the race. Illness and the occurrence of vomiting were related to the duration at sea and yacht encounter directions relative to the prevailing wind. Individual crewmember characteristics, the use of anti-motion sickness drugs, activity while at sea, and after-effects of yacht motion were also examined with respect to sickness occurrence, Sickness was greatest among females and younger crewmembers, and among crewmembers who used anti-motion sickness drugs. Sickness varied as a function of drug type and activity while at sea. Crewmembers who reported after-effects of yacht motion also reported greater sickness while at sea. ]he primary determinants of motion sickness were the duration of time spent at sea and yacht encounter direction to the prevailing wind. R ESEARCH HAS ESTABLISHED that seasickness is dependent on the magnitude of ship motion The majority of studies of seasickness have involved single voyages (e.g., 8) or multiple, short duration voyages (e.g., 10). There have been few investigations of the incidence of seasickness over prolonged periods at sea. Wiker et al. conducted 3-d sea trials involving different U.S. Coast Guard vessels navigating an octagonal course (16). The incidence of motion sickness was found to be greater on vessels producing greater magnitudes of vertical motion. Vomiting and lesser symptoms of motion sickness were greatest when steaming with a component of head seas. Applebee et al. Goto and Kanda (5) examined sickness data from 35 sea cadets on board a 97-m training vessel during a 4-month voyage in the Pacific Ocean. Motion sickness symptoms decreased logarithmically as days at sea increased, the incidence of motion sickness falling to 10% of its original value over the first 10 d at sea. The authors suggest that motion sickness incidence can be determined from two factors: a h u m a n response factor derived from the magnitude of vertical acceleration experienced, and an exposure effect function based on the decline in sickness incidence with days spent at sea. It is clear that motion sickness during extended periods at sea is not simply determined by the sea conditions. Motion sickness incidence will also be determined by the type of vessel, the vessel heading and the prevailing sea and wind conditions (encounter direction) and the period of time spent at sea. Predictions of seasickness incidence based only on short duration exposures do not allow for the effect of habituation. Initial susceptibility to motion sickness and the rate of habituation m a y differ from person to person depending on age, gender, previous sailing experience, and the use of anti-motion sickness drugs (e.g., 13). This study investigated the importance of sea state, vessel encounter direction and continuous exposure duration as determinants of motion sickness over prolonged periods at sea during the 1992-93 British Steel Challenge ocean yacht race. The effects of individual characteristics and individual behavior on the incidence of motion sickness were also examined. M E T H O D S The British Steel Challenge: T h e British Steel Challenge was a 9-month, 28,000 mile yacht race involving a cirFrom the Huma
