temperatures, may be slightly warmer than the average annual air temperature, depending on the depth of the shelter and its geographical location, but for the purpose of the following discussion it is sufficiently accurate to consider them equivalent. In many localities, shelter temperatures (equal to annual average, column 1, Table 1) would be below the dew point (column 5) in the summer. Condensation of moisture on walls and contents of shelters will occur. Column 6 shows the number of hours during the summertime (June 1 to September 30, or 2928 hours) that the dew point of the outside air is above the shelter temperature. In general for Eastern, Central, and Southern areas, the dew point is higher over 75 percent of the time. Pacific Coast areas not immediately adjacent to the sea and the Mountain States show much more favorable relationship between summer dew point and shelter temperatures. If the shelters are to hold materials for which a humidity of less than 70 percent is recommended for good storage life, the problem is greatly increased. For the effect of this requirement, the dew point data should be compared with a temperature about 10 degrees below the annual temperature. Only the desert areas have more than half of the summer hours in which the dew point is below this lower temperature. Closing a shelter tightly would, of course, prevent moisture increase from outside air. However, moisture vapor from any of the other sources listed will not have any way to get out of shelters that are not dehumidified. It is doubtful that many shelters would be built so that moisture could not get in. The build-up of moisture in most shelters, if they were shut tightly, would soon result in a relative humidity approaching 100 percent. Selective ventilation offers one way of counteracting excessive humidity if there are periods during which the humidity of the outside air is not excessive. By preventing ingress of moisture to the shelter as much as possible and running ventilating equipment on days when the humidity is favorable, it may be possible for some shelters in high humidity areas to maintain acceptable humidity levels during all or most of the time. Ventilation during the summertime will raise the shelter temperature to some extent. While an increase in temperature is favorable in counteracting the humidity problem it is detrimental to storage life of foods. In commercial warehouses, selective ventilation and heating are commonly used to counteract the increase in moisture vapor that occurs in spring and summer. On warm days of low humidity, air is circulated through the warehouse to warm up the stacks of foods. Supplementary heat may be used to increase the temperature rise of the stored materials if outside air temperature is not sufficient. By proper use of these steps, all chances of the relative humidity becoming too high can be avoided. It is doubtful if such practices will be used to an appreciable extent in shelters because of the close control required and the increase in rate of deterioration of the foods due to higher temperatures. Mechanical dehumidification offers the best means of controlling humidity. If measures are taken to prevent ingress of moisture as much as possible, dehumidification may be one of the lowest cost methods of attaining long life for food containers and other equipment and supplies that deteriorate at high moisture levels. Refrigeration and rechargeable, desiccant-types of dehumidifying equipment are possible mechanisms for control of shelter humidity. The inexpensive, household-type, refrigeration dehumidifiers are generally inoperable below 65°F. which is above normal ground temperature for most of the northern United States. Where such units will function, they will maintain relativee humidity at 60 percent or lower. The rechargeable, desiccant-type dehumidifiers are generally more bulky and may be more expensive to operate and maintain than the refrigeration type. In any event, provisions must be protected from moisture. If this cannot be accomplished within the framework of the shelter design and its maintenance program, then master containers or sealed and protected rooms can be used. These will be discussed in Chapter 3. Air contaminants can have a major influence on corrosion rates of metals. The sulfur-bearing compounds---sulfur dioxide, sulfurous acid, sulfuric acid, and hydrogen sulfide --are the most frequent causes of excessive corrosion rates. Other harmful materials are hygroscopic dusts, nitric acid, carbon dioxide, ammonium chloride, hydrochloric acid, ozone, and possibly other smog components. Air contaminants are found primarily in industrial areas although ozone and other smog materials also come from heavy concentrations of automobiles or other equipment using internal combustion engines. Salt sprays carried by ocean winds also cause abnormal rates of corrosion. Usually the areas affected are limited to those immediately adjacent to the water. The corrosion rates can change several fold in a distance of a few hundred feet. No information is available on the amounts of these contaminants that might get into shelters and cause increased deterioration rates. More frequent inspections will be justified for shelters built in industrial areas and near the seashore. Insects and rodents can constitute a hazard to goods stored in shelters. Some of the underground shelters in the most northerly states may be sufficiently cold that insects will not be a problem. Underground shelters in other areas and all above-ground shelters will have temperatures permitting the growth of insects. All the packaging should be resistant to insects and rodents. Repellants and pesticides would need to be reapplied periodically. To the extent possible, shelters should be constructed to keep vermin out. Chapter 3. STORAGE STABILITY OF FOOD Commercial Food Distribution Practices A common misconception exists that dry groceries found on market shelves are nonperishable. To the contrary, foods of all kinds are perishable, and shelter stockpiles should be selected with some knowledge of degree of perishability in order that satisfactory food will always be available and total costs will be minimal. (The problems of surveillance raised by the perishable nature of food and replacement of deteriorated supplies are discussed in Chapter 4.) The misconception concerning perishability of food arises from normal marketing and use patterns. Canned goods and dry groceries are revolved so rapidly through the grocers' shelves that their lack of stability is rarely detected. Table 2 illustrates the normal supply of various food items held in wholesale storage and in the average large, medium, and small retail store in terms of rate of stock turnover. TABLE 2 Inventory of various types of food normally carried Source: Private communication, Retail Grocers Association of |