heater aggravation

Probably mold in system, its common.

That's one of the early candidate causes. I brought it around to half a dozen places years ago, and the assessment was that it is certainly not. I had an antifungal mist treatment applied to the ventilation system to no effect (Ziebart). I had internal detailing done just in case (Ziebart) and very early on, I had the engine shampoo'd even though it was unlikely to be the case. I also avoided Krown this year so as not to obfuscate any sleuthing that I may conceive of.

I'm currently trying to focus on understanding the resistor better. What the coil material and the anchoring block material is, and whether it is plausibly introducing something into the air when heated.

Also, it was established in an earlier thread that the 97 Tercel is somewhat exceptional in that there is no sensor control of coolant to the heater. The amount of heating is controlled by changing the fraction of air that is blown by the core (perhaps according to the associated manual dial on the dashboard?). What exactly does the heater resistor do in this case? In electronics in general, there are thermistors that provide negative feedback to prevent overheating. I think that mention was made of the heater resistor controlling the fan, but I've never noticed the fan speed changing except in response to the dashboard control.

I doubt if "super dryness" has any bearing on the respirator aggravation because your car does not have AC and so there is no dehumidification function in your car. The relative humidity in the car will be the same as the ambient air. If you are not experiencing the aggravation in the ambient air, then don't waste time trying to humidify the air.

Do you experience less respiratory aggravation with the fan set on high or do you experience more? If you experience more with the fan on high, then it is unlikely that the resistor is the cause of the aggravation because the resistors are bypassed and are not in the circuit when the fan is set on high.

I agree that the prospects of humidfying the air in a car is pretty dim.

Regarding humidity, the water content of the heated air is the same as the ambient air, but the heated air is able to hold more water. Therefore, the relative humidity of the heated air is less than the ambient air. This extra capacity for water vapour is filled by absorbing moisture from human skin, lungs (when we breath in), and any exposed water. Basically, the hot air dries out anything it comes in contact with.

When air is warmed inside a normal shelter, the relative humidity rises with water absorption, which mitigates the drying effect. In a car, the air is rapidly being replaced under ventilation, which ensures that the drying effect is always at full tilt. Especially when air is directed toward a person, the air adjacent to the person never gets more humid as it would if the air was still, so the drying effect is always maximum.

It's hard to tell whether the problem is worse with fan speed. The coughing settles in gradually with lengthening exposure, and has a persistent effect (a long tail in time). So there is no indicator that varies sharply in response to fan speed. To a first degree, however, I detect sensations of respiratory aggravation that are roughly of the same order of magnitude regardless of the fan speed. If possible outgassing from heating resistor material is a factor, it may be one of many, another of which may be dry air.

Makes sense, I'll take your word for it.

Wouldn't a simple test of the humidity hypothesis would be to place a humidity meter (hygrometer?) in the cabin to see if the relative humidy in the car was actually lower? Or switch the air intake to recirculate?

Would an insulated mug of hot water placed in a cupholder, occasionally replenished by hot water from a Thermos bottle, be enough to humidify the cabin air? Does drinking any liquids help reduce the aggravation?

I really doubt if outgassing from the blower resistor is a factor (see my other post). The resistor doesn't get hot enough to really heat up the mounting material because it is mounted in the air flow from the fan, which cools it.

I think we are missing the forest for the trees here. When the pollen count is up and the vent is on outside air it brings the pollen into the car. Try running the vent on inside air and see if that doesn't help. The local weather stations are required to run a pollen index, watch it and see if you condition worsens on high pollen days. Anything that gets into the area where the vent intakes fresh air should be kept clean, and the drains open. When you are behind a diesel or a smoking car at a stop light you can close the outside air to avoid the smell. I suppose you could run an inverter and get an air cleaner and run inside air.

I don't have any doubt that the relative humidity goes down as the air heats up, simply because the "saturation vapour pressure" (which I referred to as the air's capacity to hold water) goes up while the actual water content does not. Loosely speaking, relative humidity is the ratio of the two.

Right now, I'm focusing on how to address the lower relative humidity, more as a test than a solution. After all, I only *suspect* that the dryness is a large factor respiratory aggravation. Aside from addressing the dryness, even anecdotes as to whether others have similar problems with dryness would shed some light.

I haven't tried open containers of hot fluids because when I drive standard, it can be rather unsmooth. I often have a lid on any hot fluids with openings for sipping. Not really enough to modify the humidity under constant ventilation.

Regarding switching to recirculate, I've found that it introduces a new confounding variable for the test. Basically, the plastic smells in the cabin seem to grow stronger with time (which makes sense). It's likely from all the plastic material in the cabin. It's not comfortable, and obfuscates my ability to discern whether anything from the system in ventilation mode is introducing a problem. If I can get ventilation working fine, I don't have to worry about the plastic smell accumulating in circulation mode. For my testing of ventilation, I often have the air directed at face level to maximize exposure the vent air, and to minimize exposure to air that is already in the cabin.

I'm always drinking fluids in the car...doesn't really help much, in the same manner drinking fluids doesn't help much in any dry-air environment. The air has direct contact with skin, lungs, and respiratory track; it can directly remove moisture faster than your body can resynthesize and direct moisture to exposed surfaces, even if you drink plenty.

I'm starting to doubt that too, especially after you explained how it is not mounted right on the heater.

Pollen has been suggested as a cause before, but problem is pretty consistent throughout the year. Maybe less in the summer simply because I don't use the heater then. Summer aside, if the problem was pollen, it would be constant during pollen season and wouldn't have a strong correlation with car usage, with the telltale sign of coughing bouts that follow car usage.

As I describe above, I have tried characterizing the problem in circulation mode with the associated confounding problems. To a first order, though, I would say that the problem persists.

Regarding an inverter...what is that?

Regarding air cleaner, do you mean something that plugs into the cigarette lighter and (say) rests on the floor?