Paintings are fragile and changes are to be expected.  Many changes do not threaten the stability of a painting and are not considered damage.  Paintings generally do well in environmental conditions that are comfortable for people; the structural components of a painting can expand and contract in different ways as the surrounding temperature and humidity fluctuate. 

The physical components of an oil painting expand and contract in reaction to atmospheric conditions at different speeds and in unequal proportion, subjecting the painting to stress. Prolonged exposure to extremes of dryness, humidity, heat or cold with little air flow will make a painting susceptible to structural damage.

• Panel paintings subjected to low or high relative humidity and temperatures could result in warping or splitting of the wood panel support.  The wood is also susceptible to insect infestation.
• Linen canvas subjected to high humidity will noticeably sag or loosen. This natural condition can cause buckling, draws, and support failure
  Buckling and draws; a disruption causing ridges or ripples in the canvas.

Visible stress is a result of unfavorable environmental or mechanical conditions that have developed over an extended period of time.  

• Cracks
  Stress cracks are caused by various types of pressure or impact in the ground and paint layers, eventually resulting in the cracking and flaking of the paint film, sometimes accompanied by cupping
  Cupping; aged paint loosened by cracking, with edges curling to create cup-like formations, flaking or losses may be caused by a too-damp or too-dry environment.  The individual stress cracks or fissures can form a network of straight or barely curved lines.
• Flexible canvas can become overly taut, while the more brittle paint film may crack, curl, or loosen from underlying layers.
  

A common sign of age is a darkened or yellowed surface caused by accumulated grime, nicotine, or discolored varnish.

• Old varnish can be seen as a yellow or brown cast over the painted surface, obscuring the artist’s intended colors and the balance of light and dark. 
• Grime accumulation can appear as a dull grey or brownish haze caused by soot, nicotine, or ambient dust and accretions on the surface.  Superficial grime can become imbedded in the paint film.  In some instances it can be difficult or impossible to remove.
• Paintings displayed above a mantel will be exposed to soot, heat, and environmental extremes.
• Exposure to direct sunlight or ultraviolet light will produce changes in the brilliance, hue, and balance of colors and will cause the paint to become brittle. Even the most permanent colors eventually will be affected by intense light. 

Excessive or deteriorating restorations may appear as areas of a different color or texture than the surrounding paint.

• These irregularities are caused by overzealous retouching of old damages Overpainting; the repainting is extended beyond boundaries defined by the damage , otherwise known as overpainting.
• These areas can appear abraded or skinnedSkinned; original paint removed by excessive cleaning, a sunken line caused by an improper repair of a tear, or a raised portion caused by a thick patch placed on the reverse of an old puncture or tear.

Physical damage is the most common form of damage to an oil painting. Because the painted surface is traditionally not behind glass, the painting is very vulnerable, especially in transit or storage.

• Abrasions and dent-like impressions are commonly caused by objects pressed into or rested against a painting's surface. 
• Tears, holes, or punctures in a canvas support can be caused by poor storage or handling, household accidents, or natural disasters.
• Issues ranging from soot deposits Smoke damage; deposit of partially combusted material, usually accompanied by an odor generally resulting from an open flame or residue from a fireplace or a building fire. to disfigured paint can result from fire, an open source of heat, or hot lights. Scorched paint can be seen as a bubbled or blistered Blister; a disruption causing an area to protrude from the painted surface or a separation of the paint from the ground or both layers from the support surface.
• Water exposure
  Water exposure can cause paint to become extremely unstable, resulting in paint and ground loss can result in lifting, delamination Delamination; a separation of the ground, paint and varnish layers and paint loss, wavy distortions in the canvas, mold growth, or tidelines Tidelines; staining on the front or reverse of the canvas caused by water coming into contact with the canvas and subsequently drying. A saturated canvas could tighten like a drum upon drying, causing the paint surface to crack and flake immediately.

Cleaning an oil painting should be handled by a professional. If your painting should become soiled with food, smoke or other substances, do not attempt to clean it yourself. When structural damages occur to a painting such as tears, flaking paint, cracks with lifting edges, or mold, consult a conservator to decide on possible courses of treatment.