The rattle, buzz and clatter of a two-stroke engine is unmistakable, but it isn't necessarily good for the planet. That's because A) two-strokes burn oil and B) they are inefficient. Lubricating oil has hydrocarbons and additives that, when burnt, can form particulate matter and several volatile organic compounds, which eventually end up being released into the atmosphere. Despite modern two-stroke engines using DFI (direct fuel injection) and TPI (transfer port injection) technologies and not having to premix gas and oil (autolube systems), you can't take away what is inherent to their design — burning oil. Admittedly, the REV Force two-stroke engine claims to have fixed that, but it's just a prototype.
So let's not get carried away by potentials, and focus our attention on understanding the two-strokes in your lawnmower, your neighbor's chainsaw, or your mate's boat. The vast majority of readers might be familiar with the two-stroke concept, but for the uninitiated, two-stroke designs split the "suck, squeeze, bang and blow" sequence of a four-stroke piston engine across two piston strokes. By design, these engines are smaller and less complex than a four-stroke. The common crankcase-scavenged type has no traditional camshafts as such, and instead rely on ports, namely the intake, exhaust, and transfer ports, which the piston covers and uncovers as it moves up and down the cylinder.
The inefficiency argument is primarily down to the intake charge "leaking" through the exhaust port because of something called symmetric port timing. Essentially, there is a window where fresh charge enters the combustion chamber via the transfer port when the piston hasn't fully closed/covered the exhaust port. This inefficiency was greatly improved using DFI and, later, through TPI.
With DFI and TPI, fuel is injected after the exhaust port has closed, which dramatically reduces the exhaust emissions and improves efficiency. As you can probably tell, timing is really crucial here, but it still doesn't take away the two-stroke's oil-burning characteristics. Unlike the days of yore, that's a big no-no as far as the various emissions regulations are concerned. Even though there are two-stroke lube oils that help reduce smoke and odor, having a cloudy exhaust is unlikely to be well received in today's climate-sensitive environment.
California, for example, has banned new sales of all small gas engines that are rated below 25 horsepower, which are typically the two-stroke kind powering lawnmowers and leaf blowers. But more importantly, the California Air Resources Board (CARB) restricts the sale of two-stroke dirt bikes and ATVs that don't comply with emissions to "closed-course competition use only." Even with the Trump administration striking down California's emissions standards and blocking the state's plan to the sale of new gas-only cars in 2035, it's unlikely that previous classifications will be revised.
That said, California has stated that not all two-stroke boats are banned from use. In fact, boat engines with DFI (made since 1999), which, according to CARB, are considered clean emission engines, are not prohibited from being used. Carbureted items, on the other hand, are considered high-emission engines because they dump up to 30% of unburned fuel into the atmosphere or water bodies (in the case of boats and vessels). Montana has something similar in effect, as do Virginia and Utah, although there are nuances to those laws.
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