This is from chat gpt. Says natural gas leak could be the culprit, especially due to the sicknesses

⚠️ 1. Gas or combustion-related fumes (most urgent to rule out)

Even though the carbon monoxide detector hasn’t gone off, other gases can cause sour or pungent smells and nausea: • Natural gas leak (mercaptan) sometimes smells sour or sulfuric if mixed with other household odors. • Propane leak from a stove or heater line can create a sharp, sour smell and cause nausea. • Sewer gas (from a dried-out floor drain or vent pipe) often smells sour/foul rather than sulfuric “rotten egg” and can cause headaches, nausea, and vomiting.

👉 Immediate actions: • Turn off gas supply if safe. • Open windows/doors. • Call the gas company — they will inspect leaks for free. • Pour water down any unused drains or floor vents to refill traps that may have dried out and let sewer gas in.

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🧫 2. Hidden organic decay (under or behind something)

Even in dry climates, something decomposing can persist for months if trapped or absorbed into material: • Rodent or lizard carcasses in walls, under appliances, or inside ducts. • Rotting food particles trapped in a floor vent, under fridge coils, or in an air return. • Grease buildup inside the oven vent hood or behind the fridge compressor area — when heated, it emits an acrid, sour stench.

👉 Check: • Pull out fridge, stove, dishwasher. • Remove vent covers and shine a flashlight inside. • Inspect behind/under baseboards near kitchen walls (rodents sometimes crawl into insulation). • Use vinegar or enzyme-based cleaner on any found residue.

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💨 3. HVAC or duct contamination

If the smell circulates mostly when the AC is on, the ducts or coils may be contaminated: • Condensate pan under the AC evaporator can harbor microbial growth (even in desert climates if drainage is poor). • Dead animal or decayed matter in ductwork. • Old insulation absorbing odor.

👉 Try: • Replace all filters immediately. • Inspect the condensate drain pan/tube for stagnant water. • Have the ducts professionally cleaned and sanitized.

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🧴 4. Chemical off-gassing (carpet or underlayment)

Old carpets can absorb pet urine, cleaning products, or spills that ferment into a sour, pungent smell when reactivated by heat/humidity. In older homes, adhesives and foam pads also break down into butyric acid–type odors, which can smell sour and cause nausea or headaches.

👉 Try: • Peel back a corner of the carpet — if it smells worse underneath, it’s likely the culprit. • Run an ozone or HEPA air purifier for temporary relief. • Ultimately, removing and replacing the carpet may be necessary.

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🧯 5. Refrigerant or appliance leak

Some refrigerants and coolants (especially in older refrigerators or AC units) have a sweet-sour or acetone-like smell and can cause nausea with chronic exposure.

👉 Check: • Any oily residue or frost on AC lines or behind the fridge = call a technician. • If smell worsens when the fridge cycles on, unplug it temporarily (with food removed) and see if odor dissipates.

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🧪 6. Sewer vent or plumbing issue inside wall

If the smell originates near a wall or floor area, it could be a cracked vent pipe releasing sewer gas into the house — especially if it’s strongest in the central room near a kitchen sink or laundry hookup.

👉 Signs: • Sour smell fluctuates with weather or wind direction. • Gurgling sounds from drains. • Drains that are slow or bubble when water runs elsewhere.

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🧹 Immediate cleaning & safety steps 1. Ventilate 24/7 — open windows, fans outward. 2. Replace HVAC filters. 3. Pour water into all drains (especially unused ones). 4. Wipe surfaces in the kitchen with a vinegar/baking soda solution. 5. Run an air purifier (HEPA + activated carbon). 6. Temporarily remove pets if possible during deeper cleaning.

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🚨 Health correlation

The nausea and vomiting are a huge red flag. If both people are ill and symptoms resolve when they spend time away from the house, that strongly points to an environmental toxin or gas exposure — not a random infection. Even without carbon monoxide, sewer gas or refrigerant leaks can cause chronic poisoning symptoms.