• Flue-cured tobacco: used in conventional British cigarettes
  
• Light air-cured tobacco: used in chewing tobacco
  
• Dark tobacco: used in cigarettes from France and Spain; in chewing tobacco and cigars; in bidis of India and kreteks, or clove, cigarette of Indonesia
  
• Oriental tobacco: used in European and Turkish cigarettes

Current tobacco product regulation requires cigarette manufacturers to disclose levels of tar and nicotine. More than 4,000 chemical compounds have been identified in tobacco smoke
(Tobacco products fact sheet, 2000;
United States Department of Health and Human Services [USDHHS], 2000). Of these, over 50 compounds have been identified as carcinogens and five as developmental and reproductive toxicants (National Cancer Insititute, 1999).

Nicotine is the psychoactive drug in tobacco products that causes addiction (USDHHS, 2000). It is both a stimulant and a sedative to the central nervous system (National Institute on Drug Abuse [NIDA], 2000a).

Nicotine:

• relaxes muscles
  
• normalizes mood
  
• decreases anxiety
  
• improves attention
  
• suppresses appetite
  (Leshner, 1996)

Nicotine Absorption

Nicotine is a naturally occurring alkaloid present in the many strains of tobacco leaf cultivated to produce cigarettes. Nicotine is a small molecule that is both lipid and water-soluble; therefore, it is rapidly absorbed through the skin or lining of the mouth and nose (Henningfield & Keenan, 1993). It takes 10 - 19 seconds for nicotine to pass from the cigarette to the brain (Benowitz, 1999).

The rate of nicotine absorption depends on the pH level.

Nicotine absorption is:

• enhanced in mildly alkaline (salty) environments (high pH)
  
• severely reduced in acidic (sour) environments (low pH) (Henningfield & Keenan, 1993)

Nicotine has a pH of 8.0 (Slade, 1993). Some smokeless tobacco products have an average pH of 8.35 (CDC, 1999).

The PH of tobacco smoke is usually at or below 6.0; nearly all of the smoke and nicotine chemically is combined with acidic substances which make it non-volatile and slowly absorbed by the smoker. As the smoke pH increases above 6.0, becoming more alkaline, an increasing proportion of the total smoke-nicotine occurs in "free" form, which is volatile, rapidly absorbed by the smoker, and believed to be instantly perceived as a nicotine "kick" (Bates, Jarvis, & Connolly, 1999).

Peak nicotine levels with spit tobacco products usually occur within 30 minutes, with rapid absorption occurring with the first 10 minutes (Severson & Hatsukami, 1999). Holding a pinch of snuff in the mouth for 20-30 minutes yields nicotine levels 2 to 3 times the amount of nicotine delivered by a regular-size cigarette (USDHHS, 1993).

"Freebase" Nicotine

At least one major domestic cigarette maker uses additives to boost the absorption of nicotine in cigarette smoke (USDHHS, 2000). Ammonia compounds raise the pH of nicotine in tobacco, making it more alkaline and converting it from the protonated, bound form (various nicotine salts) to the unprotonated, freebase form (USDHHS, 2000; Bates, Jarvis, & Connolly, 1999).

Freebase nicotine more readily enters the smoke stream and crosses lung and oral cavity membranes quicker than nicotine salts do (Henningfield, Radzius, & Cone, 1995; USDHHS, 2000). Some smokeless tobacco products may have up to 68.14% "freebase nicotine"
(Centers for Disease Control [CDC], 1999).