Is it safe for us to breath in the house if the portable dryer vent blows into the house?
Portable probably means electric, so yes that should be safe in the short term.
You may soon have mold problems and the like from excess moisture, if you keep doing this, but there's no immediate danger from the exhaust, as would be the case with a gas dryer which exhausts combustion products.
You could simply put clothes on hangers and set them to dry - during the winter, they dry fairly quickly just hanging on the shower curtain rod over the tub, without blowing lint around the house.
There's nothing inherently dangerous about the exhaust vent. It simply emits warm, humid air. But it also makes a mess. Clothes produce lint, and over time that lint will build up anywhere the exhaust blows.
The lint will become unmanageable soon. It is no small thing. However a bigger issue exists, the clothes are going to take considerably longer to dry. The dryer uses the relatively dry air from the house, warms it and passes it across the clothes in the dryer. That dry hot air is good at picking-up moisture from the clothes. That moisture carrying air is then expelled out doors. If you dump that air back into the house then it will be recirculated through the dryer. That moisture laden air has far less ability to pick-up moisture from the clothes. The air can only hold so much moisture. Pretty soon all you are doing is recirculating the same moisture over and over.
You can do it if it's an electric dryer, but there are some caveats.
- The lint will make an awful mess if you don't trap it. You can't just stick a screen over the dryer's exhaust port because that would cause the lint to build up inside the dryer, reducing its efficiency and creating a fire hazard. Search online for "indoor dryer vent kit" for some ideas.
- Your house may get uncomfortably warm in the summer. If you also use an air conditioner, your power bill will go up because the air conditioner will have to dispose of the heat and humidity from the dryer.
- You may get condensation on your windows in the winter.
- If you keep your dryer in a small room or closet, you'll need to provide some ventilation.
- Using scented fabric care products may result in a very powerful smell in your house.