There are basically two different ways moisture condenses during product transportation:

1. Cargo sweating:

When cargo is transported from cold or temperate climates to tropical regions, moisture in the air will condense on the surface of the cargo or its packaging.

2. Container rain:

When containers are moved from tropical or temperate climates to cold regions, moisture in the air will condense on the inner walls of the container, which is what we commonly call container rain.

Fungi, mold, and mildew will not damage goods at relative humidity below 55%.

The occurrence and impact of corrosion depend on several variable factors, including humidity and time. Due to these variable factors, the impact cannot be described at a fixed humidity, but in general, lower humidity is better.

The type of product is also a factor to consider when deciding how much moisture to remove. This also determines the number of desiccant sticks needed to be placed in the container to reduce and maintain humidity, so that the goods in the container will not be damaged or spoiled due to excessive humidity. This depends on: weather conditions during sea transport; length of sea voyage; type of cargo in the container; type of goods and packaging materials; condition of the container; sensitivity of the goods; moisture content of flooring, wood, other types of pallets or packaging materials.

This chart clearly illustrates the problem of dampness. If the container is filled with air at 90% relative humidity and 30℃, there will be 27 grams of water per cubic meter of air. If the temperature drops to 0℃, there will be only about 5 grams of water in this cubic meter of air. The difference of 22 grams of water will condense from the air, which may damage the goods.

The humidity/moisture usually present in goods depends on the type of product and the relative humidity of the surrounding air during loading. Obviously, the relative humidity of the air in tropical regions is much higher than that in temperate regions. Therefore, moisture or water vapor is also brought into the container when the goods are loaded. For example, if a container is loaded with 5 tons of furniture, the furniture itself will contain about 8% to 25% humidity, depending on the type of wood and the degree of drying treatment. This is equivalent to about 400-1250 liters of water. Some of this moisture will be released into the air inside the container, becoming an important source of moisture condensation in the container.

A 20-foot container has a volume of approximately 33 cubic meters. In one cubic meter of subtropical air with 90% relative humidity at 32℃, there are 33 grams of water, totaling 1.3 liters of water. (At the same temperature, one cubic meter of air with 55% relative humidity contains 28 grams of water. Conversely, one cubic meter of European air at 100% relative humidity and 0℃ contains only 5 grams of water, or a total of only 0.2 liters of water. At 55% relative humidity, each cubic meter contains 3 grams of water.) Once the outside air temperature drops, 1.2 liters of water will condense on the inner walls of the container or the surface of other items inside the container. At 25℃ and 90% relative humidity, the water content in the air is relatively high. At about 22℃ and 50-60% relative humidity, the water content in the air is relatively low.