Difference between Humidity and Relative Humidity

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Relative humidity is an important factor that affects the quality of pharmaceutical products.

Moisture control is a great concern in the pharmaceutical industry from product manufacturing to packaging, and from shipping to storage. Some applications for humidity and moisture control in pharmaceuticals relate to microbiological considerations, while many applications relate to processing in order to achieve correct moisture levels that will facilitate the filling of capsules, tablet forming, and packaging.

For example, gelatin capsules are often monitored closely for moisture content to allow them to be sufficiently supple for filling without cracking or breaking in the manufacturing process. Effervescent power manufacturers must maintain tight control of the plant’s humidity in order to avoid moisture regain in the powder.

Humidity and relative humidity both measure the amount of water in the air but both of these have different in the method of measurement.

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Humidity

Humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is measured in grams of water in litter of air. It is expressed as grams of moisture per cubic meter of air (g/m3). It is also known as absolute humidity.

The maximum absolute humidity of warm air at 30°C/86°F is approximately 30g of water vapor – 30g/m3. The maximum absolute humidity of cold air at 0°C/32°F is approximately 5g of water vapor – 5g/m3.

Humidity in air depends upon the atmosphere of the area and the season of the year. In the dry areas as desert, humidity remains very low while in area near ocean and rainforests it remains very high. It also remains higher in rainy season than the summer. Air can hold a limited amount of moisture or water vapour. When moisture increases above its limit, it turns into the droplets those are known as fog.

Relative Humidity

Relative humidity also measures water vapor but RELATIVE to the temperature of the air. It is expressed as the amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature. Relative humidity is measured by the hygrometer and also known as %RH.

Warm air can hold more moisture than cold air. For instance, let’s say that a certain parcel of air can hold 30 grams of water vapour per cubic meter, but has only 15 grams of water vapour per cubic meter of air when measured. We simply divide the amount of water present by the amount of water possible, so 15 divided by 30, and then multiply it by 100 to get a percent. So in this case, 15 / 30 = 0.5, multiplied by 100 gives us 50%.  So the relative humidity is 50%, which means the air contains about half of the total water vapour it could possibly hold. Now we see that when the air is colder, the same amount of water vapour will produce a higher relative humidity than if the same amount of water vapour was present in warmer air. This is because we need to consider the amount of water vapour relative to what the air can hold.

Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal.

Relative humidity is cited in weather forecasts as it affects how we “feel” temperature.

As an example, consider two containers:

Container one has a maximum volume of 30g of water and is half full- it contains 50% of its capacity.

Container two has a maximum volume of 5g of water and is three quarters full- it contains 75% of its capacity.

Container one contains four times as much water as container two, yet actually contains a lower percentage.

If we now call container one “summer” and container two “winter”, we can start to differentiate between “absolute” and “relative” humidity.

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Humidity values are usually given as Relative Humidity.

Examples:

If the air is holding half the water it could hold, it’s Relative Humidity is 50%.

If the air is holding ALL the water it can hold it is saturated and the relative humidity is 100%

If the air is holding no water, relative humidity is 0%

If it holding a quarter of the water it could hold, 25%

100% is the highest and 0% is the lowest value of relative humidity that can be found in any area

Humidity Control in Pharmaceuticals

Each the simplest form of pharmaceutical manufacturing process can involve a range of steps before the finished product is packaged and finally reaches the consumer. These may include:

  • Lab-scale development using glove boxes, small-scale process and coaters, and fluid bed driers
  • Micronising
  • Storage of powders or liquids
  • Mixing
  • Product drying
  • Tablet compression
  • Film coating
  • Sugar coating
  • Aseptic packaging
  • Blister packaging
  • Storage of the finished product

Throughout these process, the effect of ambient humidity can have detrimental effects on product quality, yield, visual appearance and shelf-life – issues that can often effectively be avoided by the careful control of humidity in the production spaces.

Manufacturing Process Millions of caplets, pills tablets and capsules are produced every year. This process involves hygroscopic powders such as the pharmaceutical components and other fillers such as starch and/or sugar. Dehumidification systems are used to prevent moisture regain throughout the manufacturing process. Dehumidified air accelerates the drying of tablet coatings. The FDA cGMP (current Good Manufacturing Process) guidelines and regulations have the impact of law in maintaining manufacturing, packaging and storage conditions for pharmaceutical products. Part 212 of the CFR cites a temperature of 72°F (+/-3°F) and a relative humidity of 30-50%RH for manufacturing areas. However, because personnel are gowned, the temperature is often specified as 68°F by the design engineer or the owner.

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