diff --git a/calculators/__init__.py b/calculators/__init__.py
new file mode 100644
index 0000000..f3a308d
--- /dev/null
+++ b/calculators/__init__.py
@@ -0,0 +1,12 @@
+from .atmosphere import standard_atmosphere, alveolar_PO2  # type: ignore
+from .tuc import estimate_tuc  # type: ignore
+from .g_force import g_loc_time  # type: ignore
+from .radiation import dose_rate  # type: ignore
+
+__all__ = [
+    "standard_atmosphere",
+    "alveolar_PO2",
+    "estimate_tuc",
+    "g_loc_time",
+    "dose_rate",
+]
\ No newline at end of file
diff --git a/calculators/atmosphere.py b/calculators/atmosphere.py
new file mode 100644
index 0000000..663e0d5
--- /dev/null
+++ b/calculators/atmosphere.py
@@ -0,0 +1,72 @@
+import math
+
+# Physical constants
+T0 = 288.15  # Sea-level standard temperature (K)
+P0 = 101_325  # Sea-level standard pressure (Pa)
+L  = 0.0065   # Temperature lapse rate in the troposphere (K/m)
+R  = 8.31447  # Universal gas constant (J/(mol·K))
+g  = 9.80665  # Acceleration due to gravity (m/s²)
+M  = 0.0289644  # Molar mass of dry air (kg/mol)
+FIO2_STANDARD = 0.2095  # Fraction of oxygen in dry air
+
+
+def _to_meters(altitude_ft: float) -> float:
+    """Convert feet to metres."""
+    return altitude_ft * 0.3048
+
+
+def standard_atmosphere(altitude_m: float | int) -> dict:
+    """Return ISA properties up to ~32 km altitude.
+
+    Parameters
+    ----------
+    altitude_m : float | int
+        Geometric height above sea level in metres.
+
+    Returns
+    -------
+    dict
+        temperature_C : Ambient temperature in °C
+        pressure_Pa   : Ambient pressure in pascals
+        density_kg_m3 : Air density in kg/m³
+        pO2_Pa        : Partial pressure of oxygen in pascals
+    """
+    altitude_m = max(0.0, float(altitude_m))
+
+    # Troposphere (0–11 km): linear temperature lapse rate
+    if altitude_m <= 11_000:
+        T = T0 - L * altitude_m
+        P = P0 * (T / T0) ** (g * M / (R * L))
+    else:
+        # Lower stratosphere isothermal layer (11–20 km)
+        T11 = T0 - L * 11_000
+        P11 = P0 * (T11 / T0) ** (g * M / (R * L))
+        P = P11 * math.exp(-g * M * (altitude_m - 11_000) / (R * T11))
+        T = T11  # remains constant in this layer
+
+    rho = P * M / (R * T)
+    pO2 = FIO2_STANDARD * P
+
+    return {
+        "temperature_C": T - 273.15,
+        "pressure_Pa": P,
+        "density_kg_m3": rho,
+        "pO2_Pa": pO2,
+    }
+
+
+def alveolar_PO2(
+    altitude_m: float | int,
+    FiO2: float = 0.21,
+    PaCO2: float = 40.0,
+    RQ: float = 0.8,
+) -> float:
+    """Compute alveolar \(\text{PAO₂}\) via the alveolar gas equation.
+
+    All pressures in mmHg here for pedagogical familiarity.
+    """
+    # Barometric pressure from ISA
+    Pb = standard_atmosphere(altitude_m)["pressure_Pa"] / 133.322  # Pa → mmHg
+    PH2O = 47.0  # Water-vapour pressure at 37 °C (mmHg)
+    PAO2 = FiO2 * (Pb - PH2O) - PaCO2 / RQ
+    return PAO2
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diff --git a/calculators/g_force.py b/calculators/g_force.py
new file mode 100644
index 0000000..38da488
--- /dev/null
+++ b/calculators/g_force.py
@@ -0,0 +1,30 @@
+def g_loc_time(Gz: float | int) -> float:
+    """Return estimated maximum exposure time in seconds before G-LOC.
+
+    Based on a simplified interpretation of the Stoll curve (USAF, 1956).
+    Works for +G\_z accelerations in the seated position, no anti-G suit.
+    """
+    Gz = float(Gz)
+
+    if Gz < 5.0:
+        return float("inf")  # Below ~5 g most subjects tolerate indefinitely
+
+    _TABLE = {
+        6.0: 60.0,
+        7.0: 30.0,
+        8.0: 15.0,
+        9.0:  5.0,
+    }
+
+    if Gz >= 9.0:
+        return _TABLE[9.0]
+
+    # Linear interpolation between available points
+    lower_key = max(k for k in _TABLE if k <= Gz)
+    upper_key = min(k for k in _TABLE if k >= Gz)
+    if lower_key == upper_key:
+        return _TABLE[lower_key]
+
+    t_low = _TABLE[lower_key]
+    t_high = _TABLE[upper_key]
+    return t_low + (t_high - t_low) * (Gz - lower_key) / (upper_key - lower_key)
\ No newline at end of file
diff --git a/calculators/radiation.py b/calculators/radiation.py
new file mode 100644
index 0000000..afeccd1
--- /dev/null
+++ b/calculators/radiation.py
@@ -0,0 +1,15 @@
+def dose_rate(altitude_ft: float | int, polar: bool = False) -> float:
+    """Return approximate cosmic-radiation dose rate at cruise altitude.
+
+    The result is in micro-Sieverts per hour (µSv/h). A very coarse linear
+    model is used for educational demonstration only:
+
+        dose = 0.1 + 0.14 × (altitude\_ft / 10 000)
+
+    with a 20 % increase applied on polar routes (>60° latitude).
+    """
+    altitude_ft = float(altitude_ft)
+    dose = 0.1 + 0.14 * (altitude_ft / 10_000)
+    if polar:
+        dose *= 1.2
+    return dose
\ No newline at end of file
diff --git a/calculators/tuc.py b/calculators/tuc.py
new file mode 100644
index 0000000..4a7ce11
--- /dev/null
+++ b/calculators/tuc.py
@@ -0,0 +1,29 @@
+import numpy as np  # type: ignore
+
+# Altitude (ft) → median Time of Useful Consciousness (s)
+# Data compiled from USAF Flight Surgeon Handbook & other sources.
+_TIME_TABLE = [
+    (18_000, 1_500),  # 25 min
+    (22_000,   600),  # 10 min
+    (25_000,   240),  # 4 min
+    (28_000,   150),  # 2.5 min
+    (30_000,    90),  # 1.5 min
+    (35_000,    45),
+    (40_000,    18),
+    (43_000,    10),
+    (50_000,    10),
+    (63_000,     0),  # Armstrong line – instantaneous
+]
+
+
+def estimate_tuc(altitude_ft: float | int) -> float:
+    """Estimate Time of Useful Consciousness (seconds) for given altitude."""
+    altitude_ft = float(altitude_ft)
+
+    alts, times = zip(*_TIME_TABLE)
+    if altitude_ft <= alts[0]:
+        return times[0]
+    if altitude_ft >= alts[-1]:
+        return times[-1]
+
+    return float(np.interp(altitude_ft, alts, times))
\ No newline at end of file
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..b5f2046
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1,2 @@
+streamlit>=1.32
+numpy>=1.26
\ No newline at end of file
diff --git a/streamlit_app.py b/streamlit_app.py
new file mode 100644
index 0000000..30d3d7c
--- /dev/null
+++ b/streamlit_app.py
@@ -0,0 +1,108 @@
+import streamlit as st  # type: ignore
+
+from calculators import (
+    standard_atmosphere,
+    alveolar_PO2,
+    estimate_tuc,
+    g_loc_time,
+    dose_rate,
+)
+
+st.set_page_config(
+    page_title="Aerospace Physiology Calculators",
+    page_icon="✈️",
+    layout="centered",
+)
+
+st.title("✈️ Aerospace Physiology Calculators")
+
+st.sidebar.header("Choose a calculator")
+calculator = st.sidebar.selectbox(
+    "Calculator", (
+        "Standard Atmosphere",
+        "Alveolar Oxygen Pressure",
+        "Time of Useful Consciousness",
+        "G-Force Tolerance",
+        "Cosmic Radiation Dose",
+    ),
+)
+
+# ----------------------------------------------------------------------------
+# 1. STANDARD ATMOSPHERE
+# ----------------------------------------------------------------------------
+if calculator == "Standard Atmosphere":
+    st.subheader("International Standard Atmosphere (ISA)")
+    alt_ft = st.slider("Altitude (ft)", 0, 60_000, 0, step=1_000)
+    alt_m = alt_ft * 0.3048
+    result = standard_atmosphere(alt_m)
+
+    st.write(f"**Temperature:** {result['temperature_C']:.2f} °C")
+    st.write(f"**Pressure:** {result['pressure_Pa'] / 100:.2f} hPa")
+    st.write(f"**Density:** {result['density_kg_m3']:.3f} kg/m³")
+    st.write(f"**O₂ Partial Pressure:** {result['pO2_Pa'] / 100:.2f} hPa")
+
+    st.caption("Calculations up to 11 km assume a linear temperature lapse rate; above this an isothermal layer is assumed.")
+
+# ----------------------------------------------------------------------------
+# 2. ALVEOLAR GAS EQUATION
+# ----------------------------------------------------------------------------
+elif calculator == "Alveolar Oxygen Pressure":
+    st.subheader("Alveolar Oxygen Pressure (PAO₂)")
+    alt_ft = st.slider("Altitude (ft)", 0, 40_000, 0, step=1_000)
+    alt_m = alt_ft * 0.3048
+
+    FiO2 = st.number_input("Inspired O₂ fraction (FiO₂)", 0.0, 1.0, 0.21, step=0.01)
+    PaCO2 = st.number_input("Arterial CO₂ (PaCO₂) [mmHg]", 20.0, 60.0, 40.0, step=1.0)
+    RQ = st.number_input("Respiratory Quotient (R)", 0.5, 1.2, 0.8, step=0.05)
+
+    p_ao2 = alveolar_PO2(alt_m, FiO2, PaCO2, RQ)
+    st.write(f"**PAO₂:** {p_ao2:.1f} mmHg")
+
+    st.caption("Alveolar gas equation: PAO₂ = FiO₂·(Pb − PH₂O) − PaCO₂/R.")
+
+# ----------------------------------------------------------------------------
+# 3. TUC
+# ----------------------------------------------------------------------------
+elif calculator == "Time of Useful Consciousness":
+    st.subheader("Time of Useful Consciousness (TUC)")
+    alt_ft = st.slider("Altitude (ft)", 10_000, 50_000, 25_000, step=1_000)
+    tuc_sec = estimate_tuc(alt_ft)
+
+    if tuc_sec == 0:
+        st.error("Instantaneous loss of consciousness expected at this altitude without pressure suit.")
+    else:
+        minutes = tuc_sec / 60
+        st.write(f"**Estimated TUC:** {tuc_sec:.0f} s (~{minutes:.1f} min)")
+
+    st.caption("Interpolated from published USAF reference values; individual tolerance varies.")
+
+# ----------------------------------------------------------------------------
+# 4. G-FORCE
+# ----------------------------------------------------------------------------
+elif calculator == "G-Force Tolerance":
+    st.subheader("G-Force Tolerance")
+    Gz = st.slider("Sustained +Gz (g)", 1.0, 9.0, 6.0, step=0.1)
+    tol = g_loc_time(Gz)
+
+    if tol == float("inf"):
+        st.success("Below ~5 g most healthy subjects tolerate indefinitely (without anti-G suit).")
+    else:
+        st.write(f"**Estimated tolerance time:** {tol:.0f} s")
+
+    st.caption("Simplified Stoll curve approximation; assumes seated posture and no countermeasures.")
+
+# ----------------------------------------------------------------------------
+# 5. RADIATION DOSE
+# ----------------------------------------------------------------------------
+else:  # Cosmic Radiation Dose
+    st.subheader("Cosmic Radiation Dose at Cruise")
+    alt_ft = st.slider("Flight altitude (ft)", 0, 45_000, 35_000, step=1_000)
+    polar = st.checkbox("Polar route (>60° latitude)")
+    dose = dose_rate(alt_ft, polar)
+
+    st.write(f"**Dose rate:** {dose:.2f} µSv/h")
+
+    st.caption("Highly simplified linear model for educational purposes only; real-world exposure depends on solar activity, latitude, and flight duration.")
+
+st.sidebar.markdown("---")
+st.sidebar.write("Developed for educational demonstration of aerospace physiology.")
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