Particle surface area is the total surface area in square centimeters (cm2) of all the particles in one milliliter(mL) of colloid. The surface area is a calculated value based on the concentration of particles (ppm) and the mean diameter of the particles. The calculation assumes the particles are spherical. Particle surface area is inversely proportional to particle size, which means for a constant concentration of silver in the particles, the surface area increases as the particle size decreases. In the chemical world, reactivity increases with increasing surface area. Therefore, the effectiveness of a colloid increases with decreasing particle size as the particle surface area increases. For this reason, particle surface area serves as a metric for comparing different colloids. In metal colloids, the increase in reactivity that derives from the increased particle surface area translates directly to the effectiveness of the colloid. Surface area is expressed in square centimeters (cm2) per milliliter (mL) of colloidal liquid and is written as (cm2/mL). For example, the particle surface area of 1 ppm of particles having a mean diameter of 10 nm is 0.423 cm2/mL, while 1 ppm of 1 nm diameter particles has a particle surface area of 4.229 cm2/mL, which is ten times the surface area of the 10 nm particles. For more details on the relationship of particle surface area as a function of concentration and particle size, see the Colloid Surface Area table. For more discussion see Comparing Colloidal Silver Products.