In high relative humidity conditions (approximately 65% and above):

• insects and moulds thrive and reproduce readily
• metals corrode
• organic materials take up moisture, swell and change shape
• different components of single objects will take up moisture at different rates and swell by different amounts - this can lead to problems such as paint lifting from timber supports
• adhesives become sticky and very attractive as a food source for moulds and insects
• emulsion surfaces on photographs swell and can readily stick together or to the glass in their frames

In very low relative humidity conditions (approximately 35% and below):

• organic materials give out the moisture they contain - this can lead to materials becoming dry and brittle or cause them to distort and split
• different components of single objects will release moisture at different rates which can lead to loosening of bonds between them
• adhesives become dry and crack, and can fail as a result
• dimensional changes will occur in paper and textiles

If fluctuations frequently occur, materials are subjected to constant movement. This can cause severe cracking, splitting and warping.

Fluctuations in temperature cause:

- Expansion and contraction. If this is uneven and/or rapid, it can lead to physical damage and distortion -this is mostly observed in metals.
- Some types of plastic, eg. vinyl records, will shrink and warp in high temperatures;
- Insects and moulds thrive and reproduce readily.
- Metals corrode.
- Organic materials take up moisture, swell and change shape.
- Adhesives become sticky and very attractive as a food source for moulds and insects.
- Gelatine emulsions on photographs swell and can readily stick to the glass in their frames or if they are stacked, they can stick together.
- Papers that are stuck down at the edges will increase in size, and expand in the middle as their edges are restricted which can lead to creasing.
- Many chemical degradation reactions require water to proceed, which may be present in the
form of relative humidity.

If fluctuations are constantly occurring the materials are being subjected to constant movement which is usually not uniform and often results in cracking, splitting and warping.
Remember – the emphasis should be on stability