Drinking and Recreational Boating Fatalities: Title and subTitle BreakA Population-Based Case-Control Study FREE

More than 43 million people reported using a motorboat in the United States in 1994,¹ and about 800 people died in 1998 from recreational boating.² Alcohol is commonly involved in drownings and other unintentional injury fatalities^{3 - 7} and is increasingly recognized as an important factor in many boating fatalities.^{8 - 9} Data from 4 states with high testing rates for 1980 to 1985 suggest that 51% of people involved in boating fatalities had a blood alcohol concentration (BAC) of at least 40 mg/dL, and 30% had a BAC higher than 100 mg/dL.^{4 ,10} Other countries such as Canada¹¹ and Finland¹² have an even higher proportion of boating fatalities linked to alcohol use.

Alcohol use while boating affects the probability not only of ending up in the water but also of survival once that happens. Because of this apparent double jeopardy, alcohol use may actually be more hazardous on a boat than in other settings, with even low BACs greatly increasing relative risk (RR).^{8 ,13 - 14} Although these and other studies^{4 ,8 ,15 - 16} suggest that alcohol increases the RR of dying while boating, this relationship has not been well quantified.

This study sought to better define the relationship between alcohol use and the RR of death while boating. We conducted a large population-based case-control study of alcohol use and recreational boating fatality risk in 2 states, Maryland and North Carolina. These states include a diversity of waterways on which recreational boating takes place. We sought to determine the magnitude of the estimated RR of dying associated with alcohol use, adjusting for known or potential risk factors for drowning and other boating deaths. We also examined whether RRs were different for passengers and operators and whether low BACs pose a significant RR.

We searched official state boating fatality records and medical examiner files in each state to identify all recreational boating deaths classified as "accidental" that occurred from 1990 to 1998 in Maryland and North Carolina. Only boating deaths that occurred from April through October (n = 403 of 502 deaths) were included in the study. Boating activity declined markedly outside these months, making control interviews prohibitively expensive and difficult. Because of difficulty finding control subjects at night, especially in North Carolina, boating deaths that occurred between midnight and 7:00 AM in Maryland and between 9:00 PM and 7:00 AM in North Carolina were excluded from the study (13.9% of eligible cases). Deaths associated with the use of sailboats, personal watercraft (ie, jet skis), and rafts were excluded (16.1% of eligible cases). Deaths on sailboats are rare, and personal watercraft and rafts are different from other boat types.^{2 ,17 - 18} Fatality and control subjects younger than 18 years (9.7% of eligible cases) were excluded because the parents of potential underage control subjects were often not available to give consent. Small inland bodies of water were excluded in Maryland, since only 3% of eligible deaths occurred in them and they were widely dispersed. Despite the Coast Guard definition of a boating death,² individuals who drowned while swimming from a boat were included in our study, although some of our analyses excluded them.

Control subjects were from a stratified random sample of boats from waterways in each state during the boating season (April through October) from 1997 through 1999. A complex sampling design was used to ensure that control subjects were drawn from the same locations as fatality subjects in each state. First, the state's navigable waterways were divided according to geographic area and type of waterway into strata that reflected cultural and demographic differences (Table 1). Within each stratum, areas were selected to represent locations of boating activity. Given the large differences in the types of waterways and their distribution in Maryland and North Carolina, sampling procedures were tailored for each state.

North Carolina. The state was first divided into 3 geographically and culturally distinct regions (coastal, midstate, and western) with historically different patterns of alcohol use.

Bodies of water were categorized into ocean/bay/sound waterways, large and medium-sized lakes, and small lakes and rivers. The ocean/bay/sound waterways were treated as 1 stratum, large and medium-sized lakes in each region composed 3 additional strata, and all smaller lakes and rivers composed the final stratum (a total of 5 strata). Within the ocean/bay/sound stratum,¹⁹ geographically distinct areas were identified, and 6 of these were randomly selected. Each of the 12 largest lakes and 6 of the 10 medium-sized lakes were randomly selected as sampling areas, with the number of medium-sized lakes selected proportionate to the population within each region. For the small lake and river stratum, the state was subdivided by latitude and longitude. Of the 210 resulting subdivisions, 42 (20%) were selected, with a probability proportionate to the population within the region. Within each selected subdivision, 2 areas, 1 small lake and 1 river, were then randomly chosen from navigable waterways, which resulted in the selection of 30 small lakes and rivers across the state, for a total of 54 selected areas in North Carolina.

Maryland. Recreational boating in Maryland occurs primarily in 4 bodies of water, with the majority occurring on the Chesapeake Bay and its many river estuaries and also on the Potomac River.

Deep Creek Lake and the Atlantic Ocean are 2 other areas where boating is popular, but neither had any boating fatalities during the study period. Chesapeake Bay was divided into 6 strata corresponding to the upper, middle, and lower sections on both the western and eastern sides of the bay. The Potomac River was divided into 3 strata, 2 below Washington, DC, and 1 nontidal part above it. Each stratum was divided into areas that could be surveyed in 1 day.

Teams of 2 interviewers visited designated areas (eg, a lake, river, or region of a bay) multiple times by boat on a predetermined schedule that included both weekdays and weekends. On each visit, interviewers moved systematically around the water to ensure that the entire area was covered. Upon arriving at a designated location, interviewers identified up to 6 boats nearest to them and then used a die to randomly select 1 to interview. Only stationary or slowly moving boats were sampled. These fell into 2 categories: those that were anchored, moored, drifting, or berthed, and those that were arriving at destinations in a sampling area such as a fishing area, beach, marina, or boat ramp after being under way. In Maryland only, because of the large size of the Chesapeake Bay, we also used shore teams to interview boaters who were returning to a boat ramp or marina. Boats were approached in the order in which they arrived at the shore-based sampling site.

The selected boat was approached and the operator was asked to participate in the study. The operator was interviewed and asked to provide details on the boat and the boat's activities in the past hour. Next, the operator and up to 2 randomly selected passengers (≥18 years) were asked to complete a short self-administered questionnaire that included questions on general health and demographic characteristics. Last, the operator and the selected passengers were asked to provide a breath sample for alcohol testing by a handheld breathalyzer (CMI Intoxilyzer D-400R; CMI Inc, Owensboro, Ky). The interviewer also recorded information about the boat, number of passengers, evidence of alcohol use, apparent sobriety of the operator, and refusals. Institutional review boards for the protection of human subjects at the Johns Hopkins School of Public Health and the University of North Carolina School of Public Health approved the study procedures.

When recovery of a body is delayed, decomposition can result in postmortem alcohol production. Rather than excluding the subjects that were not recovered within 1 or 2 days after death,¹⁹ we used a conservative procedure based on new evidence about the time course of decomposition to adjust those subjects' BAC levels (J.A.H. and G.S.S., unpublished data, 2001). The amount subtracted from the observed BAC started as 0 mg/dL for cases with a submersion time of 12 hours and increased linearly to a maximum of 40 mg/dL for bodies recovered after 96 hours in the water. Few drowning victims produce endogenous alcohol levels as high as 40 mg/dL, even at the longest recovery times.^{20 - 21} Cases in which the body was recovered more than 1 week after the incident were excluded.

The increased RR of fatality associated with BAC, after adjustment for other factors, was estimated by calculating odds ratios (ORs) using logistic regression in Stata (StataCorp, Version 6, 2000; College Station, Tex). Effects of the sampling design (stratification, clustering, and weighting) were accounted for in the analysis by using the svy Stata commands. Each area within a stratum where control boaters were sampled (eg, a lake, a section of river, or an area of bay) was treated as a primary sampling unit or cluster (Table 1). Because the number of passengers in control boats ranged from 0 to 14 but a maximum of only 2 passengers was sampled, appropriate weights were applied to provide a valid comparison of operators and passengers. All analyses were adjusted for the confounding effects of time of day (resulting from the sampling schedule) by including variables for time of day in 2-hour increments. For the main analysis, both BAC and age were treated as continuous variables. Higher-order terms were considered for both variables. Categories of BAC were created only to compare the results with findings from other studies.

Multiple imputations were carried out to replace missing values for sex, race, and age (1%, 12%, and 12%, respectively, for controls and 15% for race for subjects; Table 2). A hot-deck procedure using the approximate Bayesian bootstrap method of Rubin and Schenker^{22 - 23} was used. Ten imputations were performed for each analysis. This approach assumes that within each state (Maryland or North Carolina) and boat type, missing values for subgroups of subjects had the same distribution as known values.

Crude analyses suggested that control operators who refused to participate might have had higher BACs than participating operators; 5% and 2%, respectively, were judged to be at least moderately impaired. For operators only we evaluated the extent to which refusals to give a breath sample might have influenced BAC RR estimates. The hot-deck method described above was used to impute the missing BACs, assuming missing values for BACs had the same distribution as those with known BAC within each level of the interviewer's assessment of impairment.

Of the 253 boating victims meeting inclusion criteria, 15 (6%) were excluded from the analysis because their bodies were recovered more than 1 week after death or were recovered after an unknown length of time. Among the 238 eligible fatality subjects, 76% were recovered within 24 hours of death; 11%, within 25 to 48 hours; 9%, within 49 to 96 hours; and 4%, within 97 to 168 hours. Seventeen of these subjects (7.1%) were not tested for BAC. Of the 221 subjects included in the study, 55% had a positive BAC (adjusted for recovery time); 36% had a BAC of at least 50 mg/dL; 27%, at least 100 mg/dL; 18%, at least 150 mg/dL; 11%, at least 200 mg/dL; and 7%, at least 250 mg/dL. Most subjects had been in open motorboats at least 3 m long (69.7%), and the largest number of them died between 6:00 and 8:00 PM (20.8%; Table 2). Subjects were predominantly male and nonblack, less than half were operators, and most were 21 to 40 years of age. Eligible subjects excluded because of missing BAC data did not have different demographic factors. Eleven subjects (3.2%) died in rough water, which precluded safely interviewing control subjects in similar conditions, but because they had BACs similar to those of other subjects, they were kept in the study. Passengers were more likely than operators to have a positive BAC (68% vs 48%; P<.001) that was at least 100 mg/dL (37% vs 27%; P = .04).

The number of boats sampled from each of the 14 strata ranged from 75 to 504. Almost all (93%) of the operators of boats sampled for the control survey agreed to participate; 87% completed the self-administered questionnaire, and 86% provided a valid breath sample. Of those who gave a breath sample, 7.6% refused the self-administered questionnaire. The interviews yielded a total of 4801 potential controls (2468 operators and 2333 passengers), of whom 3943 provided a valid breath sample and were included in the analysis (Table 2). Boating and demographic characteristics of persons who provided a breath sample differed little from that of those who refused, although those on open motorboats, those who were approached earlier in the day, female subjects, and younger persons were somewhat more likely to participate. Only 17% of participants had a positive BAC. Of those, 7.4% had a BAC of at least 50 mg/dL; 3.4%, at least 100 mg/dL; 1.4%, at least 150 mg/dL; 0.6%, at least 200 mg/dL; and 0.3%, at least 250 mg/dL. These figures represent crude unweighted distributions from a stratified sample and thus are not representative of boaters in these areas.

A greater proportion of control subjects were in motorboats at least 3 m long and were female, nonblack, and 21 to 50 years of age (Table 2). The RR of death by BAC level, compared with that of subjects with a BAC of 0 mg/dL, was determined in analyses to be a second-order quadratic relationship when adjusted for age, race, sex, occupant status, boat type, location, time of day, and weekend/weekday. Age was modeled as a third-order quadratic relationship. The ORs for dying by BAC increased most rapidly at lower BACs, with the rate of increase leveling off at higher BACs (Figure 1). The RR of death was increased even at a BAC of 10 mg/dL (OR = 1.3; 95% confidence interval [CI], 1.2-1.4), increasing to an OR of 52.4 at a BAC of 250 mg/dL (95% CI, 25.9-106.1; Table 3). When only those persons meeting the official Coast Guard definition of boating accidents were considered (ie, when the 22 subjects [10%] who died while voluntarily swimming from a boat and when control subjects from boats where people were swimming were excluded), there was no significant change in the RRs of fatality (Table 3).

Additional analyses were conducted by using categories of BAC and dichotomizing BAC at different cut points to permit comparisons with other studies (Table 4). These values have wider CIs than estimates of RR when BAC is used as a continuous variable.

The RR associated with BAC was not significantly different between operators and passengers, male and female subjects, black and nonblack persons, persons of different ages, or different types of boats.

Adjusting for the potential bias resulting from control subjects who declined to give breath samples decreased the ORs, but the differences were not significant. Because subjective impressions of intoxication are unreliable, we elected to present findings based on actual measurements, as has been the practice in the few studies that have evaluated refusal bias.^{24 - 26}

The most important finding in this study is the strong positive association of BAC with the RR of death among recreational boaters aged 18 years and older, even at BACs less than 50 mg/dL. In addition, passenger and operator drinking is associated with the same increased RR of death, regardless of whether the boat is under way.

The RRs associated with alcohol use and boating fatality increase markedly as the BAC increases, from an OR of 1.3 at a BAC of 10 mg/dL to 52 at 250 mg/dL. Our finding of increased RR at low BACs is consistent with experimental studies that find significant impairment in many safety-related tasks at BACs below 50 mg/dL.^{27 - 30}

Alcohol can affect boater safety in multiple ways, influencing both the risk of ending up in the water (or crashing) and chances for survival in the water.^{8 ,13 - 14 ,29 - 31} Alcohol impairs balance and coordination, which can increase the risk of falling overboard whether a boat is under way or not. Impaired judgment resulting from an elevated BAC can also increase the likelihood of being in high-risk situations, and unlike on the roadway, having a sober operator will not necessarily protect impaired occupants. The effects of alcohol on the probability of survival are greater than for other injury causes³¹ and, once a person enters the water, include an increased risk of hypothermia and a reduced ability to keep the head above water.^{8 ,13 - 14 ,29} Thus, a simple fall overboard can prove fatal.

Although there is substantial evidence for the risk of drinking and driving,^{27 ,30 ,32 - 34} there is surprisingly little information about the risk of drinking and other injuries, including those associated with boating. Besides that reported here, the only study designed to estimate the risk of drinking for boaters was conducted at boat ramps in California. That study had a small sample size and did not control for several relevant factors such as region, time of day, age, sex, or boat type.⁴ It found a crude 10.7-fold increased risk of boating fatality among operators with BACs higher than 100 mg/dL, and CIs were wide (95% CI, 4.7-68.8). In this study, we found a clear dose-response relationship and controlled for many potential confounding variables. In addition to elevated RR at very low BACs, we also found a much greater RR of death at higher BACs than the California study reported. Our main analyses included subjects swimming or diving off a boat, since swimming is a common part of boating activities, although excluding them in accordance with Coast Guard practice² did not change the RR.

Alcohol use has long been a part of recreational boating; 30% to 40% of boaters surveyed report drinking while boating.^{1 ,6 ,35 - 38} Many of these boaters believe that they can safely drink more when at anchor or tied up and when they are passengers rather than operators.³⁶ Current legislation concentrates entirely on alcohol use by the boat operator while the boat is under way, prohibiting operation of a boat while intoxicated, as have many safety campaigns.^{8 - 9 ,15} Some have even promoted the use of a designated driver when boating, with the implication that passengers can drink as much as they like as long as the operator remains sober. Although these approaches initially appear attractive, they ignore the reality that passengers can put themselves at risk regardless of the operator's actions or alcohol use. Only about half the recreational boating fatalities could be attributed to operator error.⁸ Most boating fatalities involve drowning; only 18% involve collisions with other boats or objects. The majority of fatalities involve falling overboard, and almost half (46%) of these occur when the vessel is not under way. Indeed, our findings clearly indicate that the RR of death is similar for operators and passengers and increases for both groups as BAC increases.

Many fatalities occur in unpowered or low-powered boats,^{2 ,8 ,15} and many others occur while boats are not in operation, which undermines the assumption that boat handling by drunken operators is a primary cause of boating fatalities. Unfortunately, since boating police rarely test surviving operators for alcohol use, it is impossible with current data to assess the role of impaired operators in increasing the risk of death for other boaters.

The implicit assumption of designated driver programs—that a passenger can drink as long as the operator remains sober—is dangerous for boaters. All persons on a boat have an increased RR of mortality if they have been drinking, even at low BACs. These findings suggest that countermeasures directed only at operators of moving boats are likely to have less impact on alcohol-related boating fatalities than broader efforts to address drinking by anyone engaged in recreational boating.

Temporal changes in drinking practice among boaters could affect alcohol risk estimates, since fatality- and control-subject data were collected for different years. However, throughout the study period BACs among subjects did not change significantly over time, nor did RRs of death estimated across cases from 1990 to 1994 and from 1995 to 1998.

Although many potentially confounding variables were taken into account, we were unable to adjust for other variables that might affect risk, such as the boater's swimming ability, the operator's boating skills and experience, use of personal floatation devices, water and weather conditions, and the condition and seaworthiness of the boat. Use of personal floatation devices was low among control subjects (about 6.7% of adults in control boats), but because such use was assessed only at the boat level and not for individuals, it was impossible to include it in our analyses. However, this study was designed to look at the total RR of death when subjects had been drinking, not to separately examine the influence of BAC on the risk of falling in the water (or crashing) and surviving once in the water. Personal floatation device use and swimming ability would have a direct effect only on the latter. Finally, although we controlled for boating exposure with the random selection of control subjects, some groups, such as persons in boats that spent most of their time under way, may have been underrepresented.