Principally, paints are either a colloidal suspension of solid pigment particles or are an emulsion of dense viscous dye gel or paste with a filler all dispersed through a lighter free-flowing liquid medium — the solvent.
After a long period in storage, the dense paint pigment and filler settles out over time and it can lose some of its solvent due to evaporation, becoming so thick and viscous that it does not flow properly when used.
These solvents are volatile organic compounds (VOCs), with white or mineral spirits having a very low flash point at about 40°C (104°F), the same as some popular brands of charcoal starter.
[12] In countries with poor environmental protection regulation, workers commonly experience a high exposure to these chemicals with consequent damage to their health.
[14] [15] Due to their hazardous nature and environmental threat of damaging pollution — persistent organic pollutants from aromatic organic compounds that are resistant to degradation are often found in wastewater with poor handling and disposal resulting in them seeping into groundwater, contaminating public water supplies [16] [17] — so in recent decades, laws from legislatures like the European Parliament in EU regulations have extensively reduced the usage of these VOC solvents in favour of water-based paints — that is, using ones like acrylic paints that have been reformulated to be made with water as the primary solvent, with only low levels of hydrocarbon solvents, if any — which perform in a very similar way as oil paints, but also are much less polluting, so have a much lower environmental impact.