Smoking recovery

Negative years smoked are rejected with invalid_input so a form mistake never invents a recovery curve.

Input contract, dates, and error behaviour

POST a JSON body with three required fields. quit_date (aliases quit, cessation_date, date_quit, stopped_on) is the calendar day the person stopped, as a real Gregorian date in YYYY-MM-DD form with years from one through nine thousand nine hundred ninety-nine; impossible days such as the thirty-first of April are rejected. years_smoked (aliases years, duration_years, smoking_years, years_of_smoking) is the duration of smoking at cessation: a finite non-negative number, zero allowed, at most one hundred twenty. as_of (aliases as_of_date, reference_date, on, date) is the evaluation day, also YYYY-MM-DD, and must fall on or after the quit date so years since quit cannot go negative. On success the body echoes quit_date, as_of, and years_smoked, sets days_since_quit to the integer serial-day difference, years_since_quit to that difference divided by 365.2425, remaining_excess_fraction to the Doll–Peto remaining share, risk_reduction_pct to one hundred times one minus that share, doll_peto_k to 4.5, plus formula, message, and steps. Failed validation—missing fields, non-finite numbers, negative years smoked, a future quit relative to as_of, dates outside the supported year range, or more than one hundred twenty years since quit—returns invalid_input and is never charged. Successful calls cost a flat $0.002 with unit item.

Practical uses, limits, and how this differs from pack-years

Prefer a pack-year tool when you only need cumulative cigarette exposure; prefer this capability when the question is excess-risk recovery as a function of quit date and duration. Keep durations inside the published absolute range so the power cannot overflow. Pair examples deliberately: quit on two thousand sixteen July twenty-fifth, as_of two thousand twenty-six July twenty-fifth, twenty years smoked for a decade of recovery; forty years smoked with only five years since quit to show that longer duration slows the fractional drop; a negative years_smoked value to demonstrate the hard invalid_input path. This landing targets people searching for a smoking recovery calculator, lung cancer risk after quitting, or how much lung-cancer risk falls when you stop smoking. It is not a pack-years calculator (that product is cigarettes per day times years smoked, a different search intent), not a PLCOm2012 or LLP clinical risk score, not a cotinine converter, and not medical advice for an individual patient. Failed tasks that return invalid_input do not consume prepaid balance. Round-trip checks are easy: recompute days between the two ISO dates, divide by 365.2425, raise D over D plus T to 3.5, and confirm that one hundred times one minus that quantity equals the published risk_reduction_pct within twelve-decimal rounding.