Many of the suggestions below may also be ways to practice minimalism or to keep things simple. Some of these suggestions you might find pretty austere, but they're for your own good. :-P Just kidding. Well actually, they are for your own good, and for the good of others, but if you don't want to follow certain suggestions, there may still be other ways to improve indoor air quality (IAQ). For example, if you don't want to give up wearing perfumes or colognes, you could at least use healthy ones.
For almost all of these suggestions, I don't include any evidence or reasons that they may help improve IAQ, but if you want to find some, you could search the web. For example, you could google 'fabric softeners indoor air quality' to find evidence and reasons that using fabric softeners hurts IAQ.
Suggestions
Here are some ways to improve IAQ:
Eliminate sources of indoor air pollution
Keep your home scent-free
Don't burn things, or at least only burn them outdoors
Don't use wood-burning or gas fireplaces
Don't smoke or vape, even pure organic cannabis or tobacco
Don't burn candles (especially scented ones) or incense
Avoid secondhand exposure to smoke and vapor
Prefer unscented products if possible
Buy (or make your own) healthy, hypoallergenic, all-organic, green, eco-friendly products
Read the list of ingredients and make sure they're all healthy and organic
Don't use air fresheners or odor eliminators
Even diffusing organic essential oils may be unhealthy
Don't use fabric softeners, dryer sheets, or fabric conditioners
Don't use perfumes or colognes
Don't use deodorants
Don't use aftershaves
Don't use hairstyling products, such as gels, mousses, hairsprays, and shaping creams
Don't use moisturizers, such as balms, creams, lotions, and oils
Don't use powders, such as talcum powders, dusting powders, body powders, perfumed powders, and baby powders
Don't use nail polish or nail polish remover, or at least only use them outside or at least in the bathroom with the bathroom exhaust fan turned on until the smell's gone
Don't use permanent markers, glues, stickers, or other things that emit toxic fumes
Choose unscented trash bags
Cooking
Keep cooking to a minimum
Choose cooking methods that use lower temperatures and emit less smoke and other air pollutants
Eg, boil, steam, or pressure cook instead of sauté, stir fry, fry, deep fry, air fry, bake, broil, barbecue, grill, or smoke
Don't burn or char your food
Don't use an oven's self-cleaning feature
Choose foods that emit less air pollutants when cooked, like organic whole plants instead of processed foods, like oils, or animal products
After each toilet use, wipe the seat, rim, and floor with toilet paper to clean up any splashback or misses, close the lid, and flush
After flushing a poopy, instead of lighting matches or using air fresheners or odor eliminators, simply run the bathroom exhaust fan for a bit to clear out any smell
Don't use rugs, even in bathrooms
Choose clothing, furniture, bedding, curtains, etc made of organic natural fibers (instead of synthetic fibers)
Don't use paint, or at least choose healthy paint for your walls and repaint them sparingly
Don't use insecticides or pesticides
Don't have pets
Get a doormat and have everyone wipe the soles of their shoes clean before entering your home
Ideally also have everyone take off their shoes inside
Improve your outdoor air quality (OAQ)
Use human-powered or electric (instead of gas or rocket) technology, such as motorcycles, cars, trucks, watercraft, and aircraft; skateboards, bicycles, and scooters; and snow blowers, lawn mowers, string trimmers, and chain saws
Don't use fertilizers, eg, on your lawns
Don't use pesticides, eg, on your gardens
Don't have pools, since they may off gas air pollutants
Don't grill or smoke food
Don't burn fires
Ventilate
If your OAQ is safe, run your energy recovery ventilators (ERVs) continuously
If your OAQ is better than your IAQ and you don't have any or enough ERVs, keep your windows open
Before showering, turn on the bathroom exhaust fan and keep it on for a bit afterwards to clear out the steam and prevent mold growth
Before cooking, turn on the ducted range hood and keep it on for a bit afterwards
Clean
Clean regularly, but only use healthy cleaning products
Dust and wipe down surfaces, including ceiling fans
Wash bedding, drapery, etc
Vacuum regularly with a HEPA vacuum cleaner
Clean dryer lint screens after each load of laundry
Clean or replace all filters, such as those of your vacuums, air purifiers, air conditioners, furnaces, dehumidifiers, and ERVs
Clean all your ducts and vents, such as those of your dryer, bathroom exhaust fans, range hoods, air conditioners, furnaces, and ERVs
Choose ductless mini-split (instead of central) air conditioning with high-grade air filtration and a heat pump
Install a ductless mini-split air conditioner in every room
Install a ceiling fan in every room
Install good ventilation
An HVAC system without ventilation is just a HAC
Install at least one openable window with a screen in every room
Install a spot ERV in every room
Install a bathroom exhaust fan in every bathroom
Vent your ovens (even electric ones) to the outside
Vent your stoves (even electric ones) to the outside
Choose ducted (instead of ductless) range hoods
Keep relative humidity (RH) at healthy levels
Keep RH above 40% to kill viruses and stay healthy
Keep RH below 50% to kill dust mites and prevent mold growth
If necessary, install one or more portable or whole-house humidifiers or dehumidifiers
Choose electric (instead of gas) dryers
Choose electric (instead of gas, oil, wood, or coal) ovens
Choose electric (instead of gas, oil, wood, or coal) stoves
Choose electric or solar (instead of natural gas, propane, oil, wood, or coal) water heaters
Don't have fireplaces, or at least choose electric (instead of gas, wood, or coal) fireplaces
Don't have furnaces, or at least choose electric (instead of natural gas, propane, oil, wood, or coal) furnaces
Don't have boilers, or at least choose electric (instead of natural gas, propane, oil, wood, or coal) boilers
Codes and standards
The International Codes (I-Codes), developed by the International Code Council (ICC), especially:
International Mechanical Code (IMC)
International Residential Code (IRC)
ANSI/ASHRAE Standard 62.2: Ventilation and Acceptable Indoor Air Quality in Residential Buildings
The requirements and stretch provisions of the National Healthy Housing Standard (NHHS)
Other healthy habits
Maybe this section should be its own post, but I'm leaving it here for now.
Only use healthy cleaning products
Don't use products with harmful chemicals or synthetic dyes
Don't use chlorine bleach, non-chlorine bleach, or ammonia
Don't use hand sanitizers or antibacterial soap
Clean with just water when you can or choose healthy body washes, shampoos, hand soaps, dish soaps, dishwasher detergents, laundry detergents, all-purpose cleaners, etc
Use healthy toothpaste
When cooking, boil, bake, or steam; don't grill, fry, or sauté
Use kitchenware, water bottles, etc made of glass, stainless steel, etc (instead of plastic)
Avoid using aluminum foil and plastic wrap
Instead, eg, use parchment paper for baking and wax paper for wrapping foods
Use ethernet (instead of Wi-Fi) when available
Whether normal exposure to radio waves is unhealthy is controversial
Exercise and stretch regularly
Eat healthily
Sleep well
Dim the lights, including screens, etc, after sunset
Go to sleep at 10 PM every night, including weekends
Sleep in a dark and quiet room
Hydrate
Meditate
Use water filters
Use shower filters
Regularly clean or replace water and shower filters
Don't use suntan lotion, or at least use a healthy one, with only zinc oxide as the active ingredient
Don't use bug repellent, or at least use a healthy one