In artificial incubation, relative humidity is considered to be one of the most important factors for hatchability and chick quality. If the relative humdity during the incubation process is not up to a certain level, it is often assumed that the results, both in hatchability and in chick quality, will be far from optimum.

But how realistic are these expectations? After all, when we look at nature, mother hen cannot control the relative humdity during the incubation process but has to accept whatever the weather in that period is providing her. If relative humidity would be so crucial for the incubation process, after all these millions of years of incubation mother nature would have found a way to help mother hen to get some control over it. 

So lets take a closer look at different (assumed) factors related to relative humdity during incubation, and their possible effect on the outcome of the incubation process.

- Moisture loss (weight loss)

One of the main effects of relative humidity is that it forces an egg to lose moisture. When there is a difference in water vapor pressure between the inside and the outside of the egg, moisture (gas) will tend to go from the highest water vapor pressure through the pores and membranes of the shell towards the lowest vapor pressure. This means that the difference in water vapor pressure inside and outside, together with the resistance of the shell against passage of the water molecules (the conductance) will determine the moisture loss of the egg. Water vapor pressure is the result of relative humdity and temperature, and as during incubation the temperature inside and outside of the egg and the humidity inside of the egg is more or less fixed, the relative humidity around the egg will determine the moisture loss. The optimum moisture loss of an egg during incubation is approximately 12 to 14%, so with the relative humidity in the machine we can control this moisture loss.

But if this moisture loss needs to be so precise, than how does mother hen control it? Well, she doesn't. As she sits on a clutch of her own eggs, all the eggs in her nests have more or less the same conductance. And as long as the eggs are losing more than 6-7% or moisture and less than 19-20% of moisture during incubation, they will hatch as the air cell in the egg will become big enough for the embryo/chick to start lung ventilation. But in our machines we have an enourmous quantity of eggs with all different levels of conductance. Due to this high variability in conductance, we need to keep the average moisture loss on approximately 12% to avoid that a few eggs (with the lowest conductance) will have a moisture loss level below the minimum of 6-7%. And the average level should not be more than 14%, to avoid that a few eggs lose more than the maximum allowed level of 19-20%. So although there is a need to control the relative humidity, a slight differentation from the optimum will not result in a total disaster, as there are only a few eggs influenced (the ones with the lowest or highest levels of conductance).

- Pattern of humidity (moisture loss)

It is often believed that relative humdity needs to be constant throughout incubation, resulting in a constant, equal moisture loss for every day of the process. As the function of the weight loss is to create an air cell, and that air cell is only needed at internal pipping, it doesnt matter for the embryo when the air cell is formed, as long as it is there at the end. So only the total moisture loss at internal pipping (or more practical, at transfer as that is easier to measure) is important. That means we can have very high or very low relative humidity at any moment in time, as long as it results in an optimum weight loss at transfer or internal pipping. However, the level of relative humdity can influence the functioning of the machines, so although the eggs do not care about the pattern, the machines might.

Relative humidity and dehydration

Many people have the expectation that when the relative humdity is too low, the embryo will dehydrate. If we look at the structure of a hatching egg and the developing embryo, we see that the embryo floats in the amnion fluid, and that the allantoic membrane is a sort of a bag with fluid between the amnion and the shell. The allantoic membrane is filled with waste water from the embryo, and the moisture loss happens from this allantoic membrane. This means that the allantoic functions as a buffer between the embryo fluid (the amnion) and the shell. When the relative humidity is low and more moisture from the egg is lost, it means that more waste water is lost from the allantoic, but not necessarily from the amnion or the embryo. This makes it unlikely that the embryo will dehydrate due to low relative humdity, as it "swims" in the amniotic fluid, and it will dump it's waste water in the allantoic. This is logical, as mother nature does not let mother hen control relative humdity. If a low relative humidity would dehydrate the embryo, she would probably have found a way to control this moisture loss.

- Minimum levels of relative humidity

There is a wide belief that during incubation the relative humidity should not drop below a certain level. For some people that is 50%, for other people the minimum level might be 30 or 40%. There is no "hard" evidence for this, but many people have the impression or experience that with low levels of relative humidity especially the chick quality will be bad, with poor navels, hard bellies, red hocks and other chick quality issues. This is probably not due to the relative humidity, but due to the way how relative humdity is created (see "Relative humidity and temperature").

- Relative humidity and heat transfer

It is often assumed that relative humidity is needed to facilitate the transfer of heat from egg to air (and vice versa). This is not the case, relative humidity has no influence on heat transfer. However, it does influence the heat capacity of the air. Heat capacity means the amount of energy that is needed to warm up a specific amount of air. In moist air not only the air needs to be warmed up, but the water in the air as well, which requires more energy. A practical consequence is that when a machine is ventilated with a certain amount of air, the amount of energy that can be taken up by that air within the limits of the temperature boundaries is higher for moist air than for dry air. In that way, it might look as if moist air increases the heat transfer, as more heat is removed by the same amount of air at the same temperature difference. However, this is heat capacity and not heat transfer, eggs do not lose heat more easy when the relative humidity goes up. Besides that, the effect is very very small and hardly noticable in a commercial situation, and therefore practically not very relevant (although it should be taken into account when designing machines or calculating ventilation rates).

- Relative humidity and temperature

By itself relative humidity has no influence on the temperature of the machine, accept for the heat capacity as mentioned before. However, the evaporation of water to increase the relative humidity has an enourmous cooling effect. As many especially older machines do not have enough cooling capacity, the evaporation of water has been one of the major cooling sources in artificial incubation (next to ventilation). This is the main factor why people have the impression that a high relative humidity (especially at the end of incubation when the eggs have the highest heat production) is important for succesful incubation. At the end of incubation the ventilation is at it's max to supply the eggs with oxygen and to remove carbon dioxide. With that ventilation, also moisture is removed and the relative humidity drops. If the sprayer doesn't come in to increase the relative humidity, evaporation will not take place and the eggs will get overheated, resulting in poor chicks and poor hatchability. Although modern (single stage) machines often do have enough cooling capacity to take out the heat of the eggs by the radiators, the assumption is still there that relative humidity is needed to control the temperature in the machines.

- Relative humdity and hatching

It is often assumed that during hatching the relative humdity needs to be high. Sometimes the argument is used that it is needed to keep the membranes soft during hatching, sometimes it is even argued that the egg shell gets soft because of the high relative humidity, helping the chick to get out of the egg. This is not the case. But a lot of machines do have a problem when the ventilation during hatching is increased too much, as it makes the machine unstable and created cold spots. Due to this, the last chicks have a problem hatching and the hatching process will be "dragging", with poor hatchability, navel problems and weak chicks as a result. As a result of the need for limited ventilation, the relative humdity (and the carbon dioxide) will go up and often people will associate that with good hatchability and chick quality. More recent technologies like on-farm hatching show that this is not related with high relative humidity (or high carbon dioxide levels) but with temperature control.

- Relative humidity and bacterial contamination

Day old chick quality and especially first week mortality is related with the level of bacterial contamination in the hatchers, especially when the chicks are just hatched and still have an open navel. Although relative humidity by itself doesn't have a big effect on bacterial growth, the spraying of water in the machine to increase the relative humidity creates a perfect environment for bacterial growth, together with the high temperature and the presence of organic material. High levels of spraying, especially in a fine mist, create a perfect environment for the bacteria to penetrate the (navel of the) newly hatched chicks and cause mortality problems 3-4 days later.  

To summarize

Relative humidity dictates moisture loss, and it is important to monitor moisture loss (weight loss) on a regular base and adjust the relative humidity accordingly. The direct influence of relative humidity on the incubation process is limited, but the cooling effect of spraying water to increase the relative humidity can have a major effect on the temperature control of the machine and the eggs. When the machine has enough options to cool the eggs without evaporating water, a "dry" machine can result in lower first week mortality due to less bacterial contamination.