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Clinical Scenario 4

A 68 year old patient comes to your office for a routine visit six months following an uncomplicated myocardial infarction (MI).  He has just completed cardiac rehabilitation and is asking your advice about his proposed ongoing exercise program.  He has experienced no chest pain or shortness of breath since his MI, and his hypertension is controlled.  He would like to exercise for 30 minutes on a cycle ergometer on three days per week at a local gym and take a 60 minute outdoor walk on two other days.  However, he heard on NPR that the National Research Council has found that more people are likely to die on days when ozone pollution is bad and wonders if there is anything he can do to avoid that risk. 

1. Mortality risk of ozone exposure. It is true that non-accidental mortality rates are generally higher on days when ozone concentrations are high than when they are low.  Daily death rates increase approximately 0.5% for each 20 ppb increase in the 24-hour average ozone concentration, and available data suggest that this effect is present even at very low concentrations of ozone.  Although this suggests that ozone is responsible for many deaths in the total population, the absolute increase in risk to the average individual on any given day is quite low.  For example, a 0.5% increase in daily death rates for an individual with a 10 year average life expectancy increases the daily probability of death from 0.0001176 to 0.0001182 or approximately 6 additional deaths per day per 10 million people.  There appears to be some variability in the effect of ozone across different regions with the risk potentially being somewhat higher in some areas and somewhat lower in others.  It is also true that elevated levels of particle pollution, another air pollutant, have been observed to be associated with increased daily mortality and with adverse events in patients with underlying cardiovascular disease.
2. Benefits of exercise. The benefits of regular exercise on many aspects of physical and mental health have been well documented as have the benefits for reducing the probability of a recurrent MI in those with underlying cardiovascular disease.  This patient should be strongly encouraged to maintain a program of regular exercise to realize these well established benefits. 
3. Balanced response. The optimal solution for this patient is to continue his exercise program as planned while taking several steps that may reduce his risk from air pollution.  These steps involve reducing exposure when feasible without creating barriers to exercise, and they require the patient to become familiar with the daily Air Quality Index and with temporal patterns of air pollution in his location.  Because outdoor concentrations of ozone are almost always higher than indoor concentrations, the patient could vary his days of indoor and outdoor exercise such that he exercised indoors on days when ozone concentrations were highest and outdoors on lower concentrations days.  Ozone concentrations usually fluctuate over the course of the day in a stable pattern for a single location.  Outdoor exercise could be scheduled to occur at times of the day such as early mornings when ozone is usually low.  A similar approach could be used to jointly reduce exposure to particle pollution.  Unfortunately indoor concentrations of particles are not much lower than those encountered outdoors so indoor exercise is not as effective in reducing exposure as it is for ozone.  However, avoiding exercise at times of day when and where particle pollution levels are generally high could result in a lower risk. This includes morning and evening rush hour, locations along busy roadways, and times when there is smoke in the air from wood stoves, fireplaces, or forest fires. On the occasional days when ozone and particle pollution levels are extremely high all day, it may be prudent to avoid or reduce the intensity or duration of exercise.