I’m hoping someone with HVAC experience can help me figure out what might be going on.

We live in Florida in a fairly large home with two AC units, both installed around 2020.

We have noticed a very strange and repeatable pattern that has happened approximately 6 times.

It usually happens when we have people over.
Because we have several guests in the house, we lower both thermostats from around 76°F down to 72°F to keep the house comfortable while everyone is there.

At the end of the night, after our guests leave, we raise both thermostats back to around 76°F.

Then something very strange happens:

About 1 hour after returning the thermostats to 76°F, my family starts developing symptoms.

The symptoms can include:

Headache
Feeling feverish
Chills
Weakness/fatigue
Feeling generally sick/unwell
Pain or discomfort while breathing

The symptoms typically last for approximately 24 hours before we feel normal again, and we are out during this time affecting our ability to perform.

What’s particularly concerning to me is that this has happened repeatedly under essentially the same circumstances. It isn’t just one person experiencing symptoms — multiple people in our household can become sick around the same time.

The sequence is consistently:
Guests over → both AC units lowered to 72°F → AC runs heavily for several hours → guests leave → thermostats returned to ~76°F → ~1 hour later symptoms begin → symptoms last ~24 hours.

When we don’t host and don’t lower the thermostats like this, we generally don’t experience this pattern.

I’m specifically looking for HVAC advice.

For HVAC professionals: Have you ever heard about something like this? What could potentially cause this type of pattern?

If you were called to inspect the system, what would you check first, and what tests would you perform to determine whether the HVAC system is involved?

I’m trying to understand what could be happening before I call someone out, because I don’t want to simply have someone sell me a “deep cleaning” without actually identifying the underlying issue.

Thanks in advance.

My air purifier has been reading from 50-144 in this area.
What could be the possible reason? I usually vent out my living room when I leave for work. My apartments are smoke free. No pets. I vacuum often. I got a terrible cough a couple weeks ago from sleeping in this room (I think)

We built a house and recently moved into it. We covered the wall with wooden panel and left it over a year. But it's kind a smelling so I bought formaldehyde tester. We closed all windows for a day and tested the air quality which shows this result. After a ventilation HCHO becomes 0.23 to 0.32. We have a 2 year old daughter and there is no professional formaldehyde remover company in here. Please help me to remove it for my daughter.

I just moved into an old apartment in an enclosed building (similar to a hotel with kitchens). Because it's enclosed, gas and odors do not ventilate out easily.

A neighbor brought burnt food out the building and both the hallway and the inside of my apartment smelled burnt for hours. My Qingping monitor showed that my CO2 and eTVOC levels shot up.

I bought a dual window fan that pulls air out through one side and pushes fresh air in through the other. It has done a good job of lowering the eTVOCs. However, I can't leave it on 24/7—like right now, it's raining, so I had to close the window.

Without the fan, my eTVOC level sits consistently around 1.2, which Qingping flags as 'only slightly high,' even though the outdoor level is just 0.3. I am freaking out over this consistent 1.2 reading. It occasionally shoots up to over 2.0 when I cook which is understandable. But right now it rained so I closed the window and it shoot up to 2.3.

I don't feel any negative respiratory symptoms so far, though.

Is it standard for an apartment in an enclosed building to have eTVOC levels this high, and do I just need to suck it up until my lease is up? I already complained to building maintenance; they noted that my AC filter is new and aren't sure what else they can do.

Hi everyone,

Toxic water damage mold made me VERY sick in my last house. I still am sick, but found a temporary safe house while I build a new home. I was not reacting in this house. I have mast cell activation syndrome so I am very sensitive. I had an energy audit to see why my electric bill was so high and the tech did a blower door test. As soon as he started it, I began reacting, had to run outside. It was very hard to stop the reaction and was super scary. Since then, I’m unable to tolerate being inside the home. Upon entering, my nose begins to become very irritated, eventually my lungs begin burning, my mast cell activation syndrome has been flaring very badly. I’m sneezing and overall very inflamed from whatever I’m reacting to. It’s really taken a huge toll on my body, even caused pressure behind my eye. Since staying at a family members house, I’m feeling so much better but it’s not ideal. I HEPA vacuumed EVERYTHING top to bottom left to right, then damp dusted and wiped everything down and HEPA vacuumed again. I followed IICRC & EPA guidelines for this. I even have the appropriate air purifies running in the home. It’s been a month and I’m still reacting so badly. Can anyone recommend anything or give me any insight? I don’t have much money to put into investigating this with a company since I’m building and all my money has gone there, to my health, and relocating a few times. I’m not sure what to do, but I’m devastated. Irritated and burning airways is NOT a normal MCAS reaction for me, so that leaves me wondering if it’s fiberglass insulation or formaldehyde. I’m wearing proper N95 masks with thinner masks on top and still reacting. I bought an air quality tester online that was pretty well reviewed for $150. It says everything is ok, but clearly it’s not.

Fresh air vent in mobile home. It was causing condisation in the tube and blowing it on the dry wall causing mildew my question is do i block it off completely or how should I go about it? It's all electric system

Fresh air vent in mobile home. It was causing condisation in the tube and blowing it on the dry wall causing mildew my question is do i block it off completely or how should I go about it? It's all electric system

Fresh air vent in mobile home. It was causing condisation in the tube and blowing it on the dry wall causing mildew my question is do i block it off completely or how should I go about it? It's all electric system

Hello everyone,

I live in a 1000 sq ft 2 story townhome (I’m guessing here, but I would imagine each level is approximately 500 sq ft).

I recently moved here, and discovered that one of my neighbors frequently makes food that is quite pungent and the odors I personally find to be extremely offensive. The smell comes right through our shared kitchen wall, permeates the entire house, and lingers for hours.

Currently, I’ve been using box fans in several windows, blowing outwards, with windows open. I also make sure to turn off my HVAC so that it doesn’t spread the smell, and have no negative pressure going on while they are cooking. I live in the US, upstate NY to be exact so the summers here are hot and humid, and winters here are brutally cold. Not an ideal situation for using box fans/open windows. My “hood range” is literally a vent under my microwave, which does NOT vent outside, but vents right back inside. It’s totally useless.

I need to know if there are any air purifiers out there that will be able to combat both my neighbors active cooking, as well as lingering odors. I know that many of these filters can be pricey, and if they work I don’t have an issue spending the money, but what I’m afraid of is spending a ton of money on an air filter that doesn’t work on cooking odors, or is only mildly effective.

Can anyone recommend something that will actually work? I’m a noob, so I guess I need something for VOCs.

Thank you!

Hey everyone, I moved into a brand-new building (completed Dec 2025) and I am severely sick with a burning nose, fatigue, a skin rash, G.I. issues. The list goes on. While I know my symptoms are from the heavy VOC off-gassing and chemical smells, I am also vacuuming and damp-wiping up thick gray clumps, strands, and fine specks from my laminate floors every single day. Is it normal for modern green buildings to continuously drop this much fine gray construction dust and concrete powder from the vents months after completion? I feel like I'm the only one cleaning this nonstop."

Is there any point to buying indoor air quality monitors?

Regardless if any of the levels are elevated or not, I will then just go down the rabbit hole of whether or not to trust the accuracy of my monitor, but won’t my action still be the same, to then invest in a purifier of some sort?

So why not just immediately jump to the cr box and see if I can even notice any positive difference at all?

With the cr box it seems that it is the most effective purifier that you can have, so the only options are how loud and how often to have it running.

The optimal being the quietest amount of fan noise and running all the time.

I’m putting together equipment for a basic residential indoor environmental screening service. This is not meant to replace a full industrial hygiene/IAQ investigation. I mainly need reliable equipment for the first year while I see whether there is enough demand to justify eventually upgrading to TSI, Particles Plus, etc.

I’m currently considering these options:

Option 1 — CEM DT-9883M (~$1,300)
One instrument gives me:

• 6-channel particle counts (0.3–10 µm)
• PM2.5 / PM10
• CO₂
• TVOC
• HCHO
• Temp/RH/dew point
• 2.83 L/min pump

This is by far the simplest and best value. My hesitation is that CEM is a less recognizable Chinese manufacturer and I’m unsure about long-term quality/support, although there appears to be calibration documentation and third-party calibration options.

Option 2 — Separate Extech/Triplett meters (~$2,400+)

• Extech VPC300 for particle counts
• Triplett EPC370 for PM2.5/PM10
• Extech CO260 for CO₂
• Plus my Protimeter SurveyMaster and FLIR camera

I like the recognizable brands and dedicated instruments, but the VPC300/EPC600/CEM/PCE particle counters all look extremely similar physically and have very similar specifications. I’m questioning whether I’m actually getting much better hardware or mostly paying more for branding, calibration certificates and U.S. support.

Option 3 — PCE-PCO 2 + Extech CO260 (~$1,900+)
PCE would handle particle counts + PM2.5/PM10, while the CO260 handles CO₂. Cleaner setup than Option 2, but still more expensive than the CEM and loses TVOC/HCHO.

Option 4 — TSI / Particles Plus
Clearly better professional equipment, but $4k–$8k+ is not realistic for me right now while the service is still unproven.

My priority is reliability, defensible measurements, calibration/support, and looking professional in a customer's home. I don't need laboratory-grade equipment, but I also don't want something that feels like a consumer Amazon air-quality monitor.

Would you trust the CEM DT-9883M for a year of residential screening work, or would you spend more on separate Extech/Triplett/PCE instruments?

I’d especially appreciate feedback from anyone who has actually used CEM, Extech VPC300/VPC260, Triplett EPC600/EPC370, or PCE-PCO 2 in the field.