The active ingredient in pepper spray is capsaicin, which is derived from the fruit of plants in the genus Capsicum, including chilis in the form of oleoresin capsicum (OC). Extraction of OC from peppers requires capsicum to be finely ground, from which capsaicin is then extracted using an organic solvent such as ethanol. The solvent is then evaporated, and the remaining waxlike resin is the oleoresin capsaicin.
An emulsifier such as propylene glycol is used to suspend OC in water, and the suspension is then pressurized to make an aerosol pepper spray. Other sprays may use an alcohol (such as isopropyl alcohol) base for a more penetrating product, but a risk of fire is present if combined with a taser.
Determining the strength of pepper sprays made by different manufacturers can be confusing and difficult. Statements a company makes about their product strength are not regulated.
* The US federal government uses CRC (capsaicin and related capsaicinoids) content for regulation. CRC is the pain-producing component of the OC that produces the burning sensation. Personal pepper sprays can range from a low of 0.18% to a high of 3%. Most law enforcement pepper sprays use between 1.3% and 2%. The federal government of the United States has determined that bear attack deterrent sprays must contain at least 1.0% and not more than 2% CRC. Because the six different types of capsaicinoids under the CRC heading has different levels of potency (up to 2× on the SHU scale), the measurement does not fully represent the strength. Manufacturers do not state which particular type of capsaicinoids are used.
* Using the OC concentration is unreliable because the concentration of CRC (and potency of these compounds) can vary. Some manufacturers may show a very high percentage of OC, but the resin itself may not be spicy enough. Higher OC content only reliably implies a higher oil content, which may be undesirable as the hydrophobic oil is less able to soak and penetrate skin. Solutions of more than 5% OC may not spray properly.
* Scoville heat units (SHU) is a common indication of pepper spiciness. It does take into account the different potency of CRC compounds, but it cannot be reliably used in pepper spray because it measures the strength of the dry product, i.e. the OC resin and not what comes in the aerosol spray. As the resin is always diluted to make it spray-able, the SHU rating is not useful on its own.