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Temperature
Toads are not able to regulate their own body temperatures, and the external temperature affects the respiration rate of toads. Although there is some variation depending upon species and environment, respiration rates of toads generally increase as the temperature increases because the metabolic needs of the toad also increase as the temperature increases. The toad must acquire more oxygen, thus increasing the respiration rate.
Oxygen Concentration
Different types of water have different amounts of dissolved oxygen. A toad has the ability to absorb oxygen through its skin when submerged in the water. However, the toad is only able to absorb oxygen if there is a significant amount of oxygen dissolved in the water. Water contains between 1 and 21 percent as much oxygen as air. Higher concentrations of oxygen enable respiration via absorption. A toad in a dry environment or in low-oxygen water is able to breathe through its lungs, instead.
Surface Area
The surface area through which oxygen is absorbed is a major factor in how much oxygen can be absorbed. The greater the surface area available, the more oxygen a toad can absorb. For instance, males of the African "Hairy" frog grow hairlike projections on their legs during mating season. These increase the surface area of the skin, enabling the frog to absorb more oxygen. Increased blood flow to the skin also increases the absorption of oxygen through the skin; this sometimes happens during hibernation.
Fungus
Because toads absorb oxygen readily through their skins, many environmental factors can inhibit a toad's ability for respiration by reducing the ability of a toad to absorb oxygen. One such factor is the chytrid fungus, which is responsible for a rapid decline in amphibian populations in cool climates. This fungus grows on the skins of many amphibians, including toads. Affected toads die because they are unable to regulate the flow of water and oxygen through their skins.