Particle surface area is what determines a colloids ability to react with its environment. Reactivity increases with increasing surface area. Particle surface area can be determined by measuring the physical properties of a colloid. Because it is comprised of two important physical properties, namely, particle size and particle concentration, it serves as good metric for comparing colloids. Since it can be expressed as a single number, particle surface area can be considered a figure of merit for the effectiveness of a colloid. In this context, effectiveness is defined as the ability of the colloid to react with its environment. The higher the surface area, the more reactive the colloid, hence the more effective it is in reacting with its environment. For a comparison of commercial products based on particle surface area see the Comparison Table.