I was lucky to have one cat that never had fleas.
I have indoor cats but current one came back from the vet after staying in for surgery with fleas. I used Diatomaceous Earth and a lot of brushing. Unfortunately needed to treat the house with a spray though and I know those are particularly foul as they affect the nervous system if inhaled.
A lot of the insecticides we use can end up in the water, household cleaning products are also full of toxins but people use so much laundry stuff that they can be smelled from the opposite end of a supermarket, no-one looks at what they are using in the 'perfumes' and 'natural fragrance'. The household stuff is never featured in these articles though, the big companies wouldn't allow it.
We are blindsided into using these spot ons on pets in the same way we are sold dry food by vets. They are told to sell it as a money making excercise, or charge a lot for a prescription to buy elsewhere. When I first had a cat we used Fullers Earth for fleas, now use Diatomaceous earth if possible or consider one of the natural repellents after checking the ingredients. I won't buy dry food, even the so called prescription stuff and if I need a toxic chemical treatment I order one online but rarely need it.
Better for the pets to not use poisons on them when a pill will do. If you can pill your pet that is... gardening gloves might be useful.