CLIMATE EFFECTS ON HUMAN HEALTH ( page 4 )

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D. HUMIDITY AND PRECIPITATION EFFECTS

1. Effects of Humidity

Humidity has an important impact on mortality since it influences the body's ability to cool itself by means of evaporation of perspiration. In addition, humidity affects human comfort, and the perceived temperature by humans is largely dependent upon atm ospheric moisture content (Persinger, 1980).

The effects of low humidity can be especially dramatic in winter, when low moisture content induces stress upon the nasal-pharynx and trachea. When very cold, dry air passes through these organs, warming occurs and air temperatures in the pharynx can reac h 30deg.F. The ability of this warmer air to hold moisture increases dramatically, and moisture is extracted at a prodigious rate from the nasal passages and upper respiratory tract, leading to excessive dehydration of these organs (Richards and Marriott, 1974). This appears to increase the chance of microbial or viral infection since a rise in the viscosity of bronchial mucous seems to reduce the ability of the body to fight offending microorganisms that may enter the body from the atmosphere. This may e xplain why Green (1966) found negative correlations between relative humidity and winter absenteeism in a number of Canadian schools.

In the summer, high moisture content during hot periods can lessen the body's ability to evaporate perspiration, possibly leading to heat stress. Recent weather/mortality models developed for the National Oceanic and Atmospheric Administration indicate th at dewpoint temperature is directly related to mortality in several eastern cities when temperatures are very hot (Kalkstein, 1985). Another summer study indicated that mental well-being may also be influenced by summer relative humidity. Persinger (1975) found significant negative relationships between relative humidity and "mood scores," which represent a measure of happiness. Sanders and Brizzolara (1982) found relative humidity to be significantly related to a linear combination of three mood variable s (vigor: r = -.82; social affection: r = -.76; elation: r = -.56).

2. Effects of Precipitation

Most of the precipitation/mortality research to date has concentrated on the impact of snow and other forms of severe winter weather. Rogot and Padgett (1976) found cold weather and snow to be statistically related to deaths from stroke and heart attack-- a finding that has been corroborated by others. In a 1978 blizzard in Rhode Island, emergency room admissions for myocardial infarction rose markedly three days after the storm, and mortality from ischemic heart disease showed a large increase for a five- day period after the storm (Faiche and Rose, 1979). The authors attributed this rise to an increase in physical and psychological stress imposed by the storm. Glass and Zack (1979) concur, suggesting that an eight-day increase in deaths from ischemic hear t disease following a number of blizzards was most likely a function of after-storm activities (snow shoveling, car pushing, etc.). Interestingly, these particular death increases appeared unrelated to temperature. Males appear to be at higher risk during these storms, probably due to the greater likelihood that they will be performing more vigorous physical activity after the storm (Glass and Zack, 1979).

In an ongoing study on the effects of snow accumulation in five U.S. cities, Kalkstein (1986) has determined threshold values of accumulated snow above which mortality rates appear to rise. In New York, significant upward trends in mortality were noted th e day after snowfalls if two or more inches of snow had accumulated. In Detroit, where snow is more common, the snowfall accumulation exceeded six inches before mortality increases were noted. No significant relationship between snowfall accumulation and mortality was apparent in Chicago. Anderson and Rochard (1979) found increases in deaths from ischemic heart disease on, and for three days after, a four-inch or greater snowfall in Toronto. Major peaks in cardiovascular deaths in Minneapolis-St. Paul als o appeared to follow days with heavy snows, with the rise most rapid the day after the storm (Baker Blocker, 1982).

Summer rainfall appears to have a limited impact on mortality. Kalkstein (1986) has shown that a significant decline in mortality is experienced the day after summer precipitation events in all of five U.S. cities studied (New York, Philadelphia, Chicago, Atlanta, Detroit). The precipitation event itself might have an indirect impact, as the cooler temperatures coinciding with a summer rainfall provide relief from excessively warm weather. However, in certain specific cases, rainfall might induce increase s in mortality. Mack (1985) found that fatal automobile accidents increased in frequency during very light rain episodes (less than .01 inch) and heavy rainfalls (greater than 0.1 inch per hour).

E. FRONTAL PASSAGES, SUNSHINE, AND CLOUD COVER IMPACTS

Frontal passages may have a profound impact on well-being and mortality as large variations in weather conditions can occur in a very short time. Rapid changes in temperature have been shown to produce a number of physiological changes in the body. Rapid drops may affect blood pH, blood pressure, urinatian volume, and tissue permeability (Persinger, 1980). Outbreaks of epidemics may also be related to frontal passage. In his study of 59 years of data, Donle (1975) noticed sudden large increases in influen za outbreaks in Germany, Norway, and Switzerland often followed the passage of a surface trough. In general, these outbreaks occurred simultaneously with the influx of cold air over northern and western Europe (the passage of a surface wave is often follo wed by a rapid influx of cold air). The influenza outbreaks in Europe most frequently occurred between January and March, when cold air masses most commonly intruded over the area.

A number of studies have also found relationships between the numbers of reported migraine attacks and rapid changes in barometric pressure. Cull (1981) found fewer occurrences of attacks when barometric pressure was low. This was partially attributed to a decrease in sunshine during low-pressure intrusions, as solar radiation is a suspected triggering mechanism for migraine onset. However, a Canadian Climate Center study (1981) found that migraines were most likely to occur on days with falling pressure, rising humidity, high winds, and rapid temperature fluctuations.

Rosen (1979) cites some startling relationships between pressure changes and human well-being. He describes research that indicates that cancer mortality rates seem to increase during low-pressure fluctuations, and deaths from circulatory diseases seem to increase during high-pressure fluctuations. He notes that rapid pressure fluctuations may penetrate buildings and propagate wave energy from their source like ripples in a pond. Humans appear to be quite sensitive to such changes.

The reduction of solar radiation by cloud cover may also have effects on well-being. By increasing the brightness level, the autonomic nervous system is affected by constriction changes in the eye pupil. According to Persinger (1980), this increases the r ate of physical activity and leads to a general feeling of well-being. Wolfe (1981) notes that the sun's rays cause chemical changes in neurotransmitter or hormone synthesis in the brain, perhaps stimulating production of the hormone epinephrine, which st imulates the mind and body. Conversely, very low light intensities are often associated with states of relaxation, tiredness, and sleepiness.

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