Short answer
Usually because light, especially ultraviolet, and heat have broken down the polymer surface, and the stabilisers that were slowing this have been used up. Which plastics yellow depends on their chemistry: PVC and aromatic plastics such as polycarbonate and polystyrene-family materials do, while polyethylene and polypropylene tend to crack and chalk instead. Clean first — a grime or smoke film can look just like yellowing.
1. Light (especially ultraviolet) breaking down the polymer surface
Test: Compare a hidden face — underneath, behind a fitting, inside a battery cover — with the exposed face. A clearly whiter hidden face means light-driven yellowing.
Many polymers, including PVC, polycarbonate and polystyrene-family plastics, form yellow products when ultraviolet light breaks their chains; stabilisers slow this but are used up over time.
2. Heat and age, even in the dark
Test: Check whether parts that sit near a heat source (behind a radiator, around a lamp, inside a device) are yellower than cool ones.
Thermal oxidation yellows plastics without light, faster at higher temperature; older plastics also lose their stabilising additives.
3. Surface grime, smoke or nicotine film
Test: Clean a small hidden patch with warm water and washing-up liquid. If the patch comes back white, the colour was on the surface, not in the plastic.
Cooking fumes, smoke and handling leave a yellow-brown film that looks like yellowing.
4. Parts from different batches or materials aged differently
Test: Compare two adjacent parts that were white when new: if one yellowed and the other did not, they are different materials or grades.
Different plastics and stabiliser packages age at different rates, so an assembly that matched when new can drift apart.
Causes are ordered by how often Colourwise expects each to be the explanation — an editorial judgement, so test them all.
Kitchen fumes, cigarette smoke and years of handling leave a yellow-brown film on light-coloured plastics that is easily mistaken for ageing. Wash a small hidden patch with warm water and washing-up liquid, rinse and dry it. If the patch comes back white, clean the whole item and the problem is solved. If it stays yellow, the colour is in the plastic. Then compare faces: the underside of a keyboard, the back of a switch plate, the inside of a battery cover. A much whiter hidden face means light-driven change. Evenly yellow all over, including hidden faces, points to heat and age. Parts beside one another that aged differently point to different materials or grades.
Plastics do not all age the same way. PVC, used for window frames and cable trunking, gives off hydrogen chloride as it degrades and forms chains of alternating double bonds that absorb blue light, so it yellows and becomes brittle. Aromatic polymers — polycarbonate, polystyrene and related materials such as ABS, and polyesters — absorb ultraviolet strongly and form coloured oxidation products. Polyethylene and polypropylene, by contrast, tend to crack and chalk on weathering rather than yellow. Manufacturers add ultraviolet absorbers and light stabilisers to slow the process, and pigments such as titanium dioxide help, but the protection is used up over time and is less effective in the aromatic plastics that yellow most.
Light is not needed for yellowing. Thermal oxidation proceeds in the dark at room temperature and faster where it is warm — behind radiators, above lamps, inside electronics that run hot — which is why the parts of a device nearest its power supply often yellow first. Older plastics are more at risk because the stabilising additives in them gradually evaporate or are consumed. That makes yellowing a one-way process: slowing it is possible by keeping items cool and out of direct sun, and conservation guidance for plastics recommends low light and filtered ultraviolet for exactly that reason, but reversing it permanently is not something cleaning can do.
Popular home 'whitening' methods for old plastics involve strong oxidisers and ultraviolet; results are often temporary and can make the surface brittle. They are not covered here.
For fittings and appliances, the practical choices are replacement, painting or acceptance. Replacement parts from a UV-stabilised grade are the neatest answer for switches, sockets and trims, though a new white part beside old ones will now mismatch the other way. Paint made for plastics, applied after the preparation its instructions describe, gives an even colour on items such as garden furniture. Window frames that are yellowing while still under guarantee should go back to the installer. For collectable computers, toys and design objects, talk to a conservator before trying anything: surface treatments can damage material that is already fragile, and some collectors value original, untreated condition.
Why: The old switches have yellowed; the new one has not yet
Fix: Replace switches a room at a time, or place the new part where it is not seen beside old ones
Why: The cleaning removed surface grime, but the plastic itself is also ageing under light
Fix: Keep it out of direct sun and away from heat; accept gradual change
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Polyolefins crack rather than yellow on weathering, PVC yellows and embrittles as it loses hydrogen chloride and forms double bonds, and aromatic polymers such as polystyrene and polycarbonate form coloured oxidation products under ultraviolet.
FactStrong evidence
Hindered amine light stabilisers and ultraviolet absorbers slow polymer photodegradation but are less effective in aromatic polymers; pigments such as titanium dioxide and carbon black also give protection.
ConventionStrong evidence
Conservation guidance recommends that plastics be displayed at no more than 150 lux with ultraviolet below 75 µW/lm, noting that old plastics are at greater risk because stabilising additives may have been lost.
Source: Care of Objects Made from Rubber and Plastic — CCI Notes 15/1
FactStrong evidence
Thermal ageing, including yellowing, occurs even in the dark at room temperature.
Source: Agent of deterioration: light, ultraviolet and infrared
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.